Two dots (diacritic)

s of two dots ', placed side-by-side over or under a letter, are used in several languages for several different purposes. The most familiar to English-language speakers are the diaeresis and the umlaut, though there are numerous others. For example, in Albanian, represents a schwa. Such diacritics are also sometimes used for stylistic reasons.
In modern computer systems using Unicode, the two-dot diacritics are almost always encoded identically, having the same code point. For example, represents both o-umlaut and o-diaeresis. Their appearance in print or on screen may vary between typefaces but rarely within the same typeface.
The word ', used in linguistics and also classical scholarship, describes the form of both the umlaut diacritic and the diaeresis rather than their function and is used in those contexts to refer to either.

Uses

Diaeresis

As the "diaeresis" diacritic, it is used to mark the separation of two distinct vowels in adjacent syllables when an instance of diaeresis occurs, so as to distinguish from a digraph or diphthong. For example, in the obsolete spelling coöperate, the diaeresis reminded the reader that the word has four syllables co-op-er-ate, not three. It is used in several languages of western and southern Europe, though rarely now in English. One well-known usage is in French - the diaeresis is used in naïve, which is commonly spelled in English without the diaeresis. It is, however, obligatory in French, to show that it is pronounced rather than .

Umlaut

As the "umlaut" diacritic, it indicates a sound shift also known as umlaut in which a back vowel becomes a front vowel. It is a specific feature of German and other Germanic languages, affecting the graphemes,, and, which are modified to,, and.
It can be seen in the Sütterlin script, formerly used widely in German handwriting, in which the letter e is formed as two short parallel vertical lines very close together.

Stylistic use

The two dot diacritic is also sometimes used for purely stylistic reasons. For example, the Brontë family's surname was derived from Gaelic and had been anglicised as "Prunty", or "Brunty", but at some point, the father of the sisters, Patrick Brontë, decided on the alternative spelling with a diaeresis diacritic over the terminal to indicate that the name had two syllables.
Similarly, the "metal umlaut" is a diacritic that is sometimes used gratuitously or decoratively over letters in the names of hard rock or heavy metal bandsfor example, those of Motörhead and Mötley Crüe, and of parody bands, such as Spın̈al Tap.

Other uses by language

A double dot is also used as a diacritic in cases where it functions as neither a diaeresis nor an umlaut. In the International Phonetic Alphabet, a double dot above a letter is used for a centralized vowel, a situation more similar to umlaut than to diaeresis. In other languages it is used for vowel length, nasalization, tone, and various other uses where diaeresis or umlaut was available typographically. The IPA uses a double dot below a letter to indicate breathy voice.

Vowels

In Albanian, Tagalog, Kashubian, and Luxembourgish represents a schwa .
In Aymara, a double dot is used on for vowel length.
In the Basque dialect of Soule, represents
In the DMG romanization of Tunisian Arabic,,,,, and represent,,,, and.
In Ligurian official orthography, is used to represent the sound.
In Māori, a diaeresis was often used on computers in the past instead of the macron to indicate long vowels, as the diaeresis was relatively easy to produce on many systems, and the macron difficult or impossible.
In Seneca, are nasal vowels, though is, as in German umlaut.
In Vurës, and encode respectively and.
In the Pahawh Hmong script, a double dot is used as one of several tone marks.
The double dot was used in the early Cyrillic alphabet, which was used to write Old Church Slavonic. The modern Cyrillic Belarusian and Russian alphabets include the letter , although replacing it with the letter without the diacritic is allowed in Russian.
Since the 1870s,, has been used in the Ukrainian alphabet for iotated ; plain і is not iotated. In Udmurt, ӥ is used for uniotated, with и for iotated.
The form is common in Dutch handwriting and also occasionally used in printed text – but is a form of the digraph "ij" rather than a modification of the letter.
Komi and Udmurt use for Mid central vowel|.
The Swedish, Finnish and Estonian languages use and to represent Near-open front unrounded vowel| and Mid front unrounded vowel|
In the languages of J.R.R. Tolkien's Middle-Earth novels, a diaeresis is used to separate vowels belonging to different syllables and on final e to mark it as not a schwa or silent.
Consonants

and Malagasy are among the very few languages with a double dot on the letter "n"; in both, n̈ is the velar nasal.
In Udmurt, a double dot is also used with the consonant letters ӝ , ӟ and ӵ .
When distinction is important, Ḧ and ẍ are used for representing and in the Kurdish Kurmanji alphabet. These sounds are borrowed from Arabic.
Ẅ and ÿ: Ÿ is generally a vowel, but it is used as the consonant in Tlingit. This sound is also found in Coast Tsimshian, where it is written ẅ.
A number of languages in Vanuatu use double dots on consonants, to represent linguolabial phonemes in their orthography. Thus Araki contrasts bilabial p with linguolabial p̈ ; bilabial m with linguolabial m̈ ; and bilabial v with linguolabial v̈.
Seneca uses for.
In Arabic the letter ẗ is used in the ISO 233 transliteration for the tāʾ marbūṭah , used to mark feminine gender in nouns and adjectives.
Syriac uses a two dots above a letter, called Siyame, to indicate that the word should be understood as plural. For instance, ܒܝܬܐ means "house", while ܒܝ̈ܬܐ means "houses". The sign is used especially when no vowel marks are present, which could differentiate between the two forms. Although the origin of the Siyame is different from that of the diaeresis sign, in modern computer systems both are represented by the same Unicode character. This, however, often leads to wrong rendering of the Syriac text.
The N'Ko script, used to write the Mandé languages of West Africa uses a two-dot diacritic to represent non-native sounds. The dots are slightly larger than those used for diaeresis or umlaut.

Diacritic underneath

The IPA specifies a "subscript umlaut", for example Hindi "potter"; the ALA-LC romanization system provides for its use and is one of the main schemes to romanize Persian. The notation was used to write some Asian languages in Latin script, for example Red Karen.
The double-dot underneath a vowel is still used in Fuzhou romanization of Eastern Min to indicate a modified vowel sound; placing the modifier diacritic underneath the vowel letter makes it easier to combine it with tonal diacritics above the letter, as in the word Mìng-dĕ̤ng-ngṳ̄.

Side dots

In historical Hangul punctuation, the diacritics 〮 and 〯 , known as bangjeom or pangchŏm, were used to mark supposed tones or pitch accents. They were written to the left of a syllable in vertical writing and above a syllable in horizontal writing.

Computer encodings

In Unicode

generally treats the umlaut and the diaeresis as the same diacritic mark. Unicode refers to both as diaereses without making any distinction, although the term itself has a more precise literary meaning. For example, represents both o-umlaut and o-diaeresis, while similar codes are used to represent all such cases.
Unicode encodes a number of cases of "letter with a two dots diacritic" as precomposed characters and these are displayed below. In addition, many more symbols may be composed using the combining character facility,, that may be used with any letter or other diacritic to create a customised symbol, though only those with real-world use are shown below.
Both the combining character and the pre-composed codepoints may be regarded as an umlaut or a diaeresis according to context. Compound diacritics are possible, for example, used as a tonal marks for Hanyu Pinyin, which uses both a two dots diacritic with a caron diacritic. Conversely, when the letter to be accented is an, the diacritic replaces the tittle, thus:.
Sometimes, there's a need to distinguish between the umlaut sign and the diaeresis sign. For instance, either may appear in a German name. ISO/IEC JTC 1/SC 2/WG 2 recommends the following for these cases:

To represent the umlaut use the Combining Diaeresis
To represent the diaeresis use Combining Grapheme Joiner + Combining Diaeresis

The same advice can be found in the official Unicode FAQ.
Since version 3.2.0, Unicode also provides which can produce the older umlaut typography.
Unicode provides a combining double dot below as.
For use with the N'Ko script, there is.
Finally, Unicode encodes a free-standing two-dots character,.

Pre-Unicode

, a seven-bit code with just 95 "printable" characters, has no provision for any kind of dot diacritic. Subsequent standardisation treated ASCII as the US national variant of ISO/IEC 646: the French, German and other national variants reassigned a few code points to specific vowels with diacritics, as precomposed characters. Some of these variants also defined the sequence,backspace, as producing but few terminals supported this.
The subsequent ISO 8859-1 character encoding includes the letters ä, ë, ï, ö, ü, and their respective capital forms, as well as ÿ in lower case only, with Ÿ added in the revised edition ISO 8859-15 and Windows-1252.