Allograph
In graphemics and typography, the term allograph is used of a glyph that is a design variant of a letter or other grapheme, such as a letter, a number, an ideograph, a punctuation mark or other typographic symbol.
Graphemics
In graphemics, an obvious example in the Latin alphabet is the distinction between uppercase and lowercase letters. Allographs can vary greatly, without affecting the underlying identity of the grapheme. Even if the word "cat" is rendered as "cAt", it remains recognizable as the sequence of the three graphemes,,.Letters and other graphemes can also have significant variations that may be missed by many readers. The letter g, for example, has two common forms in different typefaces, and a wide variety in people's handwriting. A positional example of allography is the long s, a symbol which was once a widely used as a non-final allograph for the lowercase letter s. The Arabic script has particularly strong positional allography; Arabic letters have two to four allographs based on their position in the word.
Allographs can cause difficulty for character recognition, both by humans and computers. Children learning to read do not immediately realize that allographs represent the same character; the skills develop over the initial years of reading instruction. Mismatches between the allographs used in reading and writing may inhibit students' ability to recognize and name letters. Computerized optical character recognition systems also encounter difficulties with allograph recognition, similar to human difficulties. Many different character recognition algorithms have been developed to alleviate the allograph problem for different input methods, different languages, and different users.
A further complication of allographs is that a grapheme variant can acquire a separate meaning in a specialized writing system. Two symbols that are allographic in one setting may represent different meanings in another. For example, in the International Phonetic Alphabet used in linguistics, and represent different sounds, even though they are allographs of a lower-case in normal English usage.
Such variants have distinct code points in Unicode and thus are not allographs for some applications. Because they have separate code points, even allographs like upper- versus lower-case letters may be treated as different characters by some computer applications.
Typography
In typography, the term 'allograph' is used more specifically to describe the different representations of the same grapheme or character in different typefaces. The resulting glyphs may look quite different in shape and style from the reference character or each other, but nevertheless their meaning remains the same.In Unicode, a given character is allocated a code point: all allographs of that character have the same code point and thus the essential meaning is retained irrespective of font choice at time of printing or display. Typically, for example, is given a loop tail in serif typefaces but not in sans-serif faces but its code point is constant and its meaning persists irrespective of typeface.