Unicode is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard is maintained by the Unicode Consortium, and as of 2020, there is a repertoire of covering 154 modern and historic scripts, as well as multiple symbol sets and emoji. The character repertoire of the Unicode Standard is synchronized with ISO/IEC 10646, and both are code-for-code identical.
The Unicode Standard consists of a set of code charts for visual reference, an encoding method and set of standard character encodings, a set of reference data files, and a number of related items, such as character properties, rules for normalization, decomposition, collation, rendering, and bidirectional text display order.
Unicode's success at unifying character sets has led to its widespread and predominant use in the internationalization and localization of computer software. The standard has been implemented in many recent technologies, including modern operating systems, XML, Java, and the.NET Framework.
Unicode can be implemented by different character encodings. The Unicode standard defines UTF-8, UTF-16, and UTF-32, and several other encodings are in use. The most commonly used encodings are UTF-8, UTF-16, and UCS-2 ; GB18030 is standardized in China and implements Unicode fully, while not an official Unicode standard.
UTF-8, the dominant encoding on the World Wide Web uses one byte for the first 128 code points, and up to 4 bytes for other characters. The first 128 Unicode code points represent the ASCII characters, which means that any ASCII text is also a UTF-8 text.
UCS-2 uses two bytes for each character but can only encode the first 65,536 code points, the so-called Basic Multilingual Plane. With 1,112,064 possible Unicode code points corresponding to characters on 17 planes, and with over 143,000 code points defined as of version 13.0, UCS-2 is only able to represent less than half of all encoded Unicode characters. Therefore, UCS-2 is outdated, though still widely used in software. UTF-16 extends UCS-2, by using the same 16-bit encoding as UCS-2 for the Basic Multilingual Plane, and a 4-byte encoding for the other planes. As long as it contains no code points in the reserved range U+D800–U+DFFF, a UCS-2 text is valid UTF-16 text.
UTF-32 uses four bytes to encode any given codepoint, but not necessarily any given user-perceived character, since a user-perceived character may be represented by a grapheme cluster. Like UCS-2, the number of bytes per codepoint is fixed, facilitating character indexing; but unlike UCS-2, UTF-32 is able to encode all Unicode code points. However, because each character uses four bytes, UTF-32 takes significantly more space than other encodings, and is not widely used. Examples of UTF-32 also being variable-length, while in a different sense include: "Devanagari kshi is encoded by 4 code points Flag emojis are also grapheme clusters and composed of two code point characters – for example, the flag of Japan" and all "combining character sequences are graphemes, but there are other sequences of code points that are as well; for example is one."
Origin and developmentUnicode has the explicit aim of transcending the limitations of traditional character encodings, such as those defined by the ISO/IEC 8859 standard, which find wide usage in various countries of the world but remain largely incompatible with each other. Many traditional character encodings share a common problem in that they allow bilingual computer processing, but not multilingual computer processing.
Unicode, in intent, encodes the underlying characters—graphemes and grapheme-like units—rather than the variant glyphs for such characters. In the case of Chinese characters, this sometimes leads to controversies over distinguishing the underlying character from its variant glyphs.
In text processing, Unicode takes the role of providing a unique code point—a number, not a glyph—for each character. In other words, Unicode represents a character in an abstract way and leaves the visual rendering to other software, such as a web browser or word processor. This simple aim becomes complicated, however, because of concessions made by Unicode's designers in the hope of encouraging a more rapid adoption of Unicode.
The first 256 code points were made identical to the content of ISO/IEC 8859-1 so as to make it trivial to convert existing western text. Many essentially identical characters were encoded multiple times at different code points to preserve distinctions used by legacy encodings and therefore, allow conversion from those encodings to Unicode without losing any information. For example, the "fullwidth forms" section of code points encompasses a full duplicate of the Latin alphabet because Chinese, Japanese, and Korean fonts contain two versions of these letters, "fullwidth" matching the width of the CJK characters, and normal width. For other examples, see duplicate characters in Unicode.
HistoryBased on experiences with the Xerox Character Code Standard since 1980, the origins of Unicode date to 1987, when Joe Becker from Xerox with Lee Collins and Mark Davis from Apple, started investigating the practicalities of creating a universal character set. With additional input from Peter Fenwick and Dave Opstad, Joe Becker published a draft proposal for an "international/multilingual text character encoding system in August 1988, tentatively called Unicode". He explained that "he name 'Unicode' is intended to suggest a unique, unified, universal encoding".
In this document, entitled Unicode 88, Becker outlined a 16-bit character model:
Unicode is intended to address the need for a workable, reliable world text encoding. Unicode could be roughly described as "wide-body ASCII" that has been stretched to 16 bits to encompass the characters of all the world's living languages. In a properly engineered design, 16 bits per character are more than sufficient for this purpose.
His original 16-bit design was based on the assumption that only those scripts and characters in modern use would need to be encoded:
Unicode gives higher priority to ensuring utility for the future than to preserving past antiquities. Unicode aims in the first instance at the characters published in modern text, whose number is undoubtedly far [|below] 214 = 16,384. Beyond those modern-use characters, all others may be defined to be obsolete or rare; these are better candidates for private-use registration than for congesting the public list of generally useful Unicodes.
In early 1989, the Unicode working group expanded to include Ken Whistler and Mike Kernaghan of Metaphor, Karen Smith-Yoshimura and Joan Aliprand of RLG, and Glenn Wright of Sun Microsystems, and in 1990, Michel Suignard and Asmus Freytag from Microsoft and Rick McGowan of NeXT joined the group. By the end of 1990, most of the work on mapping existing character encoding standards had been completed, and a final review draft of Unicode was ready.
The Unicode Consortium was incorporated in California on 3 January 1991, and in October 1991, the first volume of the Unicode standard was published. The second volume, covering Han ideographs, was published in June 1992.
In 1996, a surrogate character mechanism was implemented in Unicode 2.0, so that Unicode was no longer restricted to 16 bits. This increased the Unicode codespace to over a million code points, which allowed for the encoding of many historic scripts and thousands of rarely used or obsolete characters that had not been anticipated as needing encoding. Among the characters not originally intended for Unicode are rarely used Kanji or Chinese characters, many of which are part of personal and place names, making them rarely used, but much more essential than envisioned in the original architecture of Unicode.
The Microsoft TrueType specification version 1.0 from 1992 used the name Apple Unicode instead of Unicode for the Platform ID in the naming table.
Unicode ConsortiumThe Unicode Consortium is a nonprofit organization that coordinates Unicode's development. Full members include most of the main computer software and hardware companies with any interest in text-processing standards, including Adobe, Apple, Facebook, Google, IBM, Microsoft, Netflix, and SAP SE.
Over the years several countries or government agencies have been members of the Unicode Consortium. Presently only the Ministry of Endowments and Religious Affairs is a full member with voting rights.
The Consortium has the ambitious goal of eventually replacing existing character encoding schemes with Unicode and its standard Unicode Transformation Format schemes, as many of the existing schemes are limited in size and scope and are incompatible with multilingual environments.
Scripts coveredUnicode covers almost all scripts in current use today.
A total of 154 scripts are included in the latest version of Unicode, although there are still scripts that are not yet encoded, particularly those mainly used in historical, liturgical, and academic contexts. Further additions of characters to the already encoded scripts, as well as symbols, in particular for mathematics and music, also occur.
The Unicode Roadmap Committee maintain the list of scripts that are candidates or potential candidates for encoding and their tentative code block assignments on the page of the Unicode Consortium Web site. For some scripts on the Roadmap, such as Jurchen and Khitan small script, encoding proposals have been made and they are working their way through the approval process. For others scripts, such as Mayan and Rongorongo, no proposal has yet been made, and they await agreement on character repertoire and other details from the user communities involved.
Some modern invented scripts which have not yet been included in Unicode or which do not qualify for inclusion in Unicode due to lack of real-world use are listed in the ConScript Unicode Registry, along with unofficial but widely used Private Use Areas code assignments.
There is also a Medieval Unicode Font Initiative focused on special Latin medieval characters. Part of these proposals have been already included into Unicode.
The , a project run by Deborah Anderson at the University of California, Berkeley was founded in 2002 with the goal of funding proposals for scripts not yet encoded in the standard. The project has become a major source of proposed additions to the standard in recent years.
VersionsUnicode is developed in conjunction with the International Organization for Standardization and shares the character repertoire with ISO/IEC 10646: the Universal Character Set. Unicode and ISO/IEC 10646 function equivalently as character encodings, but The Unicode Standard contains much more information for implementers, covering—in depth—topics such as bitwise encoding, collation and rendering. The Unicode Standard enumerates a multitude of character properties, including those needed for supporting bidirectional text. The two standards do use slightly different terminology.
The Unicode Consortium first published The Unicode Standard in 1991, and has published new versions on a regular basis since then. The latest version of the Unicode Standard, version 13.0, was released in March 2020, and is available in electronic format from the consortium's website. The last version of the standard that was published completely in book form was version 5.0 in 2006, but since version 5.2 the core specification of the standard has been published as a print-on-demand paperback. The entire text of each version of the standard, including the core specification, standard annexes and code charts, is freely available in PDF format on the Unicode website.
In April 2020, Unicode announced that the release of the forthcoming version 14.0 had been postponed by six months from its initial release of March 2021 due to the COVID-19 pandemic.
Thus far, the following major and minor versions of the Unicode standard have been published. Update versions, which do not include any changes to character repertoire, are signified by the third number and are omitted in the table below.
Architecture and terminologyThe Unicode Standard defines a codespace of numerical values ranging from 0 through 10FFFF16, called code points and denoted as U+0000 through U+10FFFF, respectively. Out of these 216 + 220 defined code points, the code points from U+D800 through U+DFFF, which are used to encode surrogate pairs in UTF-16, are reserved by the Standard and may not be used to encode valid characters, resulting in a net total of 216 − 211 + 220 = 1,112,064 possible code points corresponding to valid Unicode characters. Not all of these code points necessarily correspond to visible characters; several, for example, are assigned to control codes such as carriage return.
Code point planes and blocksThe Unicode codespace is divided into seventeen planes, numbered 0 to 16:
All code points in the BMP are accessed as a single code unit in UTF-16 encoding and can be encoded in one, two or three bytes in UTF-8. Code points in Planes 1 through 16 are accessed as surrogate pairs in UTF-16 and encoded in four bytes in UTF-8.
Within each plane, characters are allocated within named blocks of related characters. Although blocks are an arbitrary size, they are always a multiple of 16 code points and often a multiple of 128 code points. Characters required for a given script may be spread out over several different blocks.
General Category propertyEach code point has a single General Category property. The major categories are denoted: Letter, Mark, Number, Punctuation, Symbol, Separator and Other. Within these categories, there are subdivisions. In most cases other properties must be used to sufficiently specify the characteristics of a code point. The possible General Categories are:
Code points in the range U+D800–U+DBFF are known as high-surrogate code points, and code points in the range U+DC00–U+DFFF are known as low-surrogate code points. A high-surrogate code point followed by a low-surrogate code point form a surrogate pair in UTF-16 to represent code points greater than U+FFFF. These code points otherwise cannot be used.
A small set of code points are guaranteed never to be used for encoding characters, although applications may make use of these code points internally if they wish. There are sixty-six of these noncharacters: U+FDD0–U+FDEF and any code point ending in the value FFFE or FFFF. The set of noncharacters is stable, and no new noncharacters will ever be defined. Like surrogates, the rule that these cannot be used is often ignored, although the operation of the byte order mark assumes that U+FFFE will never be the first code point in a text.
Excluding surrogates and noncharacters leaves 1,111,998 code points available for use.
Private-use code points are considered to be assigned characters, but they have no interpretation specified by the Unicode standard so any interchange of such characters requires an agreement between sender and receiver on their interpretation. There are three private-use areas in the Unicode codespace:
- Private Use Area: U+E000–U+F8FF,
- Supplementary Private Use Area-A: U+F0000–U+FFFFD,
- Supplementary Private Use Area-B: U+100000–U+10FFFD.
Format characters are characters that do not have a visible appearance, but may have an effect on the appearance or behavior of neighboring characters. For example, and may be used to change the default shaping behavior of adjacent characters. There are 163 format characters in Unicode 13.0.
Sixty-five code points are reserved as control codes, and correspond to the C0 and C1 control codes defined in ISO/IEC 6429. U+0009, U+000A, and U+000D are widely used in Unicode-encoded texts. In practice the C1 code points are often improperly-translated as the legacy Windows-1252 characters used by some English and Western European texts.
Graphic characters, format characters, control code characters, and private use characters are known collectively as assigned characters. Reserved code points are those code points which are available for use, but are not yet assigned. As of Unicode 13.0 there are 830,606 reserved code points.
Abstract charactersThe set of graphic and format characters defined by Unicode does not correspond directly to the repertoire of abstract characters that is representable under Unicode. Unicode encodes characters by associating an abstract character with a particular code point. However, not all abstract characters are encoded as a single Unicode character, and some abstract characters may be represented in Unicode by a sequence of two or more characters. For example, a Latin small letter "i" with an ogonek, a dot above, and an acute accent, which is required in Lithuanian, is represented by the character sequence U+012F, U+0307, U+0301. Unicode maintains a list of uniquely named character sequences for abstract characters that are not directly encoded in Unicode.
All graphic, format, and private use characters have a unique and immutable name by which they may be identified. This immutability has been guaranteed since Unicode version 2.0 by the Name Stability policy. In cases where the name is seriously defective and misleading, or has a serious typographical error, a formal alias may be defined, and applications are encouraged to use the formal alias in place of the official character name. For example, has the formal alias, and has the formal alias.
Ready-made versus composite charactersUnicode includes a mechanism for modifying characters that greatly extends the supported glyph repertoire. This covers the use of combining diacritical marks that may be added after the base character by the user. Multiple combining diacritics may be simultaneously applied to the same character. Unicode also contains precomposed versions of most letter/diacritic combinations in normal use. These make conversion to and from legacy encodings simpler, and allow applications to use Unicode as an internal text format without having to implement combining characters. For example, é can be represented in Unicode as U+0065 followed by U+0301, but it can also be represented as the precomposed character U+00E9. Thus, in many cases, users have multiple ways of encoding the same character. To deal with this, Unicode provides the mechanism of canonical equivalence.
An example of this arises with Hangul, the Korean alphabet. Unicode provides a mechanism for composing Hangul syllables with their individual subcomponents, known as Hangul Jamo. However, it also provides 11,172 combinations of precomposed syllables made from the most common jamo.
The CJK characters currently have codes only for their precomposed form. Still, most of those characters comprise simpler elements, so in principle Unicode could have decomposed them as it did with Hangul. This would have greatly reduced the number of required code points, while allowing the display of virtually every conceivable character. A similar idea is used by some input methods, such as Cangjie and Wubi. However, attempts to do this for character encoding have stumbled over the fact that Chinese characters do not decompose as simply or as regularly as Hangul does.
A set of radicals was provided in Unicode 3.0, but the Unicode standard warns against using ideographic description sequences as an alternate representation for previously encoded characters:
LigaturesMany scripts, including Arabic and Devanāgarī, have special orthographic rules that require certain combinations of letterforms to be combined into special ligature forms. The rules governing ligature formation can be quite complex, requiring special script-shaping technologies such as ACE, which became the proof of concept for OpenType, Graphite, or AAT.
Instructions are also embedded in fonts to tell the operating system how to properly output different character sequences. A simple solution to the placement of combining marks or diacritics is assigning the marks a width of zero and placing the glyph itself to the left or right of the left sidebearing. A mark handled this way will appear over whatever character precedes it, but will not adjust its position relative to the width or height of the base glyph; it may be visually awkward and it may overlap some glyphs. Real stacking is impossible, but can be approximated in limited cases. Generally this approach is only effective in monospaced fonts, but may be used as a fallback rendering method when more complex methods fail.
Standardized subsetsSeveral subsets of Unicode are standardized: Microsoft Windows since Windows NT 4.0 supports WGL-4 with 656 characters, which is considered to support all contemporary European languages using the Latin, Greek, or Cyrillic script. Other standardized subsets of Unicode include the Multilingual European Subsets:
MES-1, MES-2 and MES-3A & MES-3B. Note that MES-2 includes every character in MES-1 and WGL-4.
|01||00–13, 14–15, 16–2B, 2C–2D, 2E–4D, 4E–4F, 50–7E, 7F||Latin Extended-A|
|01||8F, 92, B7, DE-EF, FA–FF||Latin Extended-B|
|02||18–1B, 1E–1F||Latin Extended-B|
|02||59, 7C, 92||IPA Extensions|
|02||BB–BD, C6, C7, C9, D6, D8–DB, DC, DD, DF, EE||Spacing Modifier Letters|
|03||74–75, 7A, 7E, 84–8A, 8C, 8E–A1, A3–CE, D7, DA–E1||Greek|
|04||00–5F, 90–91, 92–C4, C7–C8, CB–CC, D0–EB, EE–F5, F8–F9||Cyrillic|
|1E||02–03, 0A–0B, 1E–1F, 40–41, 56–57, 60–61, 6A–6B, 80–85, 9B, F2–F3||Latin Extended Additional|
|1F||00–15, 18–1D, 20–45, 48–4D, 50–57, 59, 5B, 5D, 5F–7D, 80–B4, B6–C4, C6–D3, D6–DB, DD–EF, F2–F4, F6–FE||Greek Extended|
|20||13–14, 15, 17, 18–19, 1A–1B, 1C–1D, 1E, 20–22, 26, 30, 32–33, 39–3A, 3C, 3E, 44, 4A||General Punctuation|
|20||7F, 82||Superscripts and Subscripts|
|20||A3–A4, A7, AC, AF||Currency Symbols|
|21||05, 13, 16, 22, 26, 2E||Letterlike Symbols|
|21||90–93, 94–95, A8||Arrows|
|22||00, 02, 03, 06, 08–09, 0F, 11–12, 15, 19–1A, 1E–1F, 27–28, 29, 2A, 2B, 48, 59, 60–61, 64–65, 82–83, 95, 97||Mathematical Operators|
|23||02, 0A, 20–21, 29–2A||Miscellaneous Technical|
|25||00, 02, 0C, 10, 14, 18, 1C, 24, 2C, 34, 3C, 50–6C||Box Drawing|
|25||80, 84, 88, 8C, 90–93||Block Elements|
|25||A0–A1, AA–AC, B2, BA, BC, C4, CA–CB, CF, D8–D9, E6||Geometric Shapes|
|26||3A–3C, 40, 42, 60, 63, 65–66, 6A, 6B||Miscellaneous Symbols|
|F0||Private Use Area|
|FB||01–02||Alphabetic Presentation Forms|
Rendering software which cannot process a Unicode character appropriately often displays it as an open rectangle, or the Unicode "replacement character", to indicate the position of the unrecognized character. Some systems have made attempts to provide more information about such characters. Apple's Last Resort font will display a substitute glyph indicating the Unicode range of the character, and the SIL International's Unicode Fallback font will display a box showing the hexadecimal scalar value of the character.
Mapping and encodingsSeveral mechanisms have been specified for storing a series of code points as a series of bytes.
Unicode defines two mapping methods: the Unicode Transformation Format encodings, and the Universal Coded Character Set encodings. An encoding maps the range of Unicode code points to sequences of values in some fixed-size range, termed code units. All UTF encodings map code points to a unique sequence of bytes. The numbers in the names of the encodings indicate the number of bits per code unit or the number of bytes per code unit. UTF-8 and UTF-16 are the most commonly used encodings. UCS-2 is an obsolete subset of UTF-16; UCS-4 and UTF-32 are functionally equivalent.
UTF encodings include:
- UTF-1, a retired predecessor of UTF-8, maximizes compatibility with ISO 2022, no longer part of The Unicode Standard
- UTF-7, a 7-bit encoding sometimes used in e-mail, often considered obsolete
- UTF-8, uses one to four bytes for each code point, maximizes compatibility with ASCII
- UTF-EBCDIC, similar to UTF-8 but designed for compatibility with EBCDIC
- UTF-16, uses one or two 16-bit code units per code point, cannot encode surrogates
- UTF-32, uses one 32-bit code unit per code point
The UCS-2 and UTF-16 encodings specify the Unicode Byte Order Mark for use at the beginnings of text files, which may be used for byte ordering detection. The BOM, code point U+FEFF has the important property of unambiguity on byte reorder, regardless of the Unicode encoding used; U+FFFE does not equate to a legal character, and U+FEFF in other places, other than the beginning of text, conveys the zero-width non-break space.
The same character converted to UTF-8 becomes the byte sequence
EF BB BF. The Unicode Standard allows that the BOM "can serve as signature for UTF-8 encoded text where the character set is unmarked". Some software developers have adopted it for other encodings, including UTF-8, in an attempt to distinguish UTF-8 from local 8-bit code pages. However, the UTF-8 standard, recommends that byte order marks be forbidden in protocols using UTF-8, but discusses the cases where this may not be possible. In addition, the large restriction on possible patterns in UTF-8 means that it should be possible to distinguish UTF-8 from other character encodings without relying on the BOM.
In UTF-32 and UCS-4, one 32-bit code unit serves as a fairly direct representation of any character's code point. In the other encodings, each code point may be represented by a variable number of code units. UTF-32 is widely used as an internal representation of text in programs, since every Unix operating system that uses the gcc compilers to generate software uses it as the standard "wide character" encoding. Some programming languages, such as Seed7, use UTF-32 as internal representation for strings and characters. Recent versions of the Python programming language may also be configured to use UTF-32 as the representation for Unicode strings, effectively disseminating such encoding in high-level coded software.
Punycode, another encoding form, enables the encoding of Unicode strings into the limited character set supported by the ASCII-based Domain Name System. The encoding is used as part of IDNA, which is a system enabling the use of Internationalized Domain Names in all scripts that are supported by Unicode. Earlier and now historical proposals include UTF-5 and UTF-6.
GB18030 is another encoding form for Unicode, from the Standardization Administration of China. It is the official character set of the People's Republic of China. BOCU-1 and SCSU are Unicode compression schemes. The April Fools' Day RFC of 2005 specified two parody UTF encodings, UTF-9 and UTF-18.
Operating systemsUnicode has become the dominant scheme for internal processing and storage of text. Although a great deal of text is still stored in legacy encodings, Unicode is used almost exclusively for building new information processing systems. Early adopters tended to use UCS-2 and later moved to UTF-16, as this was the least disruptive way to add support for non-BMP characters. The best known such system is Windows NT, which uses UTF-16 as the sole internal character encoding. The Java and.NET bytecode environments, macOS, and KDE also use it for internal representation. Partial support for Unicode can be installed on Windows 9x through the Microsoft Layer for Unicode.
UTF-8 has become the main storage encoding on most Unix-like operating systems because it is a relatively easy replacement for traditional extended ASCII character sets. UTF-8 is also the most common Unicode encoding used in HTML documents on the World Wide Web.
Multilingual text-rendering engines which use Unicode include Uniscribe and DirectWrite for Microsoft Windows, ATSUI and Core Text for macOS, and Pango for GTK+ and the GNOME desktop.
Input methodsBecause keyboard layouts cannot have simple key combinations for all characters, several operating systems provide alternative input methods that allow access to the entire repertoire.
ISO/IEC 14755, which standardises methods for entering Unicode characters from their code points, specifies several methods. There is the Basic method, where a beginning sequence is followed by the hexadecimal representation of the code point and the ending sequence. There is also a screen-selection entry method specified, where the characters are listed in a table in a screen, such as with a character map program.
Online tools for finding the code point for a known character include Unicode Lookup by Jonathan Hedley and Shapecatcher by Benjamin Milde. In Unicode Lookup, one enters a search key, and a list of corresponding characters with their code points is returned. In Shapecatcher, based on Shape context, one draws the character in a box and a list of characters approximating the drawing, with their code points, is returned.
The adoption of Unicode in email has been very slow. Some East Asian text is still encoded in encodings such as ISO-2022, and some devices, such as mobile phones, still cannot correctly handle Unicode data. Support has been improving, however. Many major free mail providers such as Yahoo, Google, and Microsoft support it.
WebAll W3C recommendations have used Unicode as their document character set since HTML 4.0. Web browsers have supported Unicode, especially UTF-8, for many years. There used to be display problems resulting primarily from font related issues; e.g. v 6 and older of Microsoft Internet Explorer did not render many code points unless explicitly told to use a font that contains them.
Although syntax rules may affect the order in which characters are allowed to appear, XML documents, by definition, comprise characters from most of the Unicode code points, with the exception of:
- most of the C0 control codes,
- the permanently unassigned code points D800–DFFF,
- FFFE or FFFF.
말should display on all browsers as Δ, Й, ק,م, ๗, あ, 叶, 葉, and 말.
When specifying URIs, for example as URLs in HTTP requests, non-ASCII characters must be percent-encoded.
FontsUnicode is not in principle concerned with fonts per se, seeing them as implementation choices. Any given character may have many allographs, from the more common bold, italic and base letterforms to complex decorative styles. A font is "Unicode compliant" if the glyphs in the font can be accessed using code points defined in the Unicode standard. The standard does not specify a minimum number of characters that must be included in the font; some fonts have quite a small repertoire.
Free and retail fonts based on Unicode are widely available, since TrueType and OpenType support Unicode. These font formats map Unicode code points to glyphs, but TrueType font is restricted to 65,535 glyphs.
Thousands of fonts exist on the market, but fewer than a dozen fonts—sometimes described as "pan-Unicode" fonts—attempt to support the majority of Unicode's character repertoire. Instead, Unicode-based fonts typically focus on supporting only basic ASCII and particular scripts or sets of characters or symbols. Several reasons justify this approach: applications and documents rarely need to render characters from more than one or two writing systems; fonts tend to demand resources in computing environments; and operating systems and applications show increasing intelligence in regard to obtaining glyph information from separate font files as needed, i.e., font substitution. Furthermore, designing a consistent set of rendering instructions for tens of thousands of glyphs constitutes a monumental task; such a venture passes the point of diminishing returns for most typefaces.
NewlinesUnicode partially addresses the newline problem that occurs when trying to read a text file on different platforms. Unicode defines a large number of characters that conforming applications should recognize as line terminators.
In terms of the newline, Unicode introduced and. This was an attempt to provide a Unicode solution to encoding paragraphs and lines semantically, potentially replacing all of the various platform solutions. In doing so, Unicode does provide a way around the historical platform dependent solutions. Nonetheless, few if any Unicode solutions have adopted these Unicode line and paragraph separators as the sole canonical line ending characters. However, a common approach to solving this issue is through newline normalization. This is achieved with the Cocoa text system in Mac OS X and also with W3C XML and HTML recommendations. In this approach every possible newline character is converted internally to a common newline. In other words, the text system can correctly treat the character as a newline, regardless of the input's actual encoding.
Philosophical and completeness criticismshas become one of the most controversial aspects of Unicode, despite the presence of a majority of experts from all three regions in the Ideographic Research Group, which advises the Consortium and ISO on additions to the repertoire and on Han unification.
Unicode has been criticized for failing to separately encode older and alternative forms of kanji which, critics argue, complicates the processing of ancient Japanese and uncommon Japanese names. This is often due to the fact that Unicode encodes characters rather than glyphs. Unification of glyphs leads to the perception that the languages themselves, not just the basic character representation, are being merged. There have been several attempts to create alternative encodings that preserve the stylistic differences between Chinese, Japanese, and Korean characters in opposition to Unicode's policy of Han unification. An example of one is TRON.
Although the repertoire of fewer than 21,000 Han characters in the earliest version of Unicode was largely limited to characters in common modern usage, Unicode now includes more than 92,000 Han characters, and work is continuing to add thousands more historic and dialectal characters used in China, Japan, Korea, Taiwan, and Vietnam.
Modern font technology provides a means to address the practical issue of needing to depict a unified Han character in terms of a collection of alternative glyph representations, in the form of Unicode variation sequences. For example, the Advanced Typographic tables of OpenType permit one of a number of alternative glyph representations to be selected when performing the character to glyph mapping process. In this case, information can be provided within plain text to designate which alternate character form to select.
characters shown with and without italics
If the difference in the appropriate glyphs for two characters in the same script differ only in the italic, Unicode has generally unified them, as can be seen in the comparison between Russian and Serbian characters at right, meaning that the differences are displayed through smart font technology or manually changing fonts.
Mapping to legacy character setsUnicode was designed to provide code-point-by-code-point round-trip format conversion to and from any preexisting character encodings, so that text files in older character sets can be converted to Unicode and then back and get back the same file, without employing context-dependent interpretation. That has meant that inconsistent legacy architectures, such as combining diacritics and precomposed characters, both exist in Unicode, giving more than one method of representing some text. This is most pronounced in the three different encoding forms for Korean Hangul. Since version 3.0, any precomposed characters that can be represented by a combining sequence of already existing characters can no longer be added to the standard in order to preserve interoperability between software using different versions of Unicode.
Injective mappings must be provided between characters in existing legacy character sets and characters in Unicode to facilitate conversion to Unicode and allow interoperability with legacy software. Lack of consistency in various mappings between earlier Japanese encodings such as Shift-JIS or EUC-JP and Unicode led to round-trip format conversion mismatches, particularly the mapping of the character JIS X 0208 '～', heavily used in legacy database data, to either or .
Some Japanese computer programmers objected to Unicode because it requires them to separate the use of and, which was mapped to 0x5C in JIS X 0201, and a lot of legacy code exists with this usage. The separation of these characters exists in ISO 8859-1, from long before Unicode.
Indic scriptss such as Tamil and Devanagari are each allocated only 128 code points, matching the ISCII standard. The correct rendering of Unicode Indic text requires transforming the stored logical order characters into visual order and the forming of ligatures out of components. Some local scholars argued in favor of assignments of Unicode code points to these ligatures, going against the practice for other writing systems, though Unicode contains some Arabic and other ligatures for backward compatibility purposes only. Encoding of any new ligatures in Unicode will not happen, in part because the set of ligatures is font-dependent, and Unicode is an encoding independent of font variations. The same kind of issue arose for the Tibetan script in 2003 when the Standardization Administration of China proposed encoding 956 precomposed Tibetan syllables, but these were rejected for encoding by the relevant ISO committee.
Thai alphabet support has been criticized for its ordering of Thai characters. The vowels เ, แ, โ, ใ, ไ that are written to the left of the preceding consonant are in visual order instead of phonetic order, unlike the Unicode representations of other Indic scripts. This complication is due to Unicode inheriting the Thai Industrial Standard 620, which worked in the same way, and was the way in which Thai had always been written on keyboards. This ordering problem complicates the Unicode collation process slightly, requiring table lookups to reorder Thai characters for collation. Even if Unicode had adopted encoding according to spoken order, it would still be problematic to collate words in dictionary order. E.g., the word "perform" starts with a consonant cluster "สด", the vowel แ-, in spoken order would come after the ด, but in a dictionary, the word is collated as it is written, with the vowel following the ส.
Combining charactersCharacters with diacritical marks can generally be represented either as a single precomposed character or as a decomposed sequence of a base letter plus one or more non-spacing marks. For example, ḗ and ḗ should be rendered identically, both appearing as an e with a macron and acute accent, but in practice, their appearance may vary depending upon what rendering engine and fonts are being used to display the characters. Similarly, underdots, as needed in the romanization of Indic, will often be placed incorrectly.. Unicode characters that map to precomposed glyphs can be used in many cases, thus avoiding the problem, but where no precomposed character has been encoded the problem can often be solved by using a specialist Unicode font such as Charis SIL that uses Graphite, OpenType, or AAT technologies for advanced rendering features.
AnomaliesThe Unicode standard has imposed rules intended to guarantee stability. Depending on the strictness of a rule, a change can be prohibited or allowed. For example, a "name" given to a code point cannot and will not change. But a "script" property is more flexible, by Unicode's own rules. In version 2.0, Unicode changed many code point "names" from version 1. At the same moment, Unicode stated that from then on, an assigned name to a code point will never change anymore. This implies that when mistakes are published, these mistakes cannot be corrected, even if they are trivial. In 2006 a list of anomalies in character names was first published, and, as of April 2017, there were 94 characters with identified issues, for example:
- : This is a small letter. The capital is
- : Does not join graphemes.
- : This is not a Yi syllable, but a Yi iteration mark.
- : bracket is spelled incorrectly.