Glyph
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A glyph is a carved figure or character, incised or in relief; a carved pictograph; hence, a pictograph representing a form originally adopted for sculpture, whether carved or painted. Augustan English scholars of the early 18th century, imitating French antiquaries, adopted glyph from the Greek word meaning a "carving." Compare the carved and incised "sacred glyphs" hieroglyphs, which have had a longer history in English dating from the first Elizabethan translation of Plutarch (who adapted "hieroglyphic" as a Latin adjective). But "glyph" first came to widespread European attention with the engravings and lithographs from Frederick Catherwood's drawings of undeciphered glyphs of the Maya civilization in the early 1840s. "Glyphs" still bring connotations of Maya glyphs to mind.
In typography, a glyph is a graphical representation of a character, sometimes several characters or only a part of character. The actual glyphs in typography were originally the carved and cast characters of a font. A character is a textual unit, whereas a glyph is a graphical unit. For example, the sequence ffi contains three characters, but will be represented by one glyph in TeX, since the three characters will be combined into a single ligature. Conversely, some typewriters require the use of multiple glyphs to depict a single punctuation mark (for example, two hyphens in place of a dash, or an overstruck apostrophe and period in place of an exclamation mark).
In the simple case, for a given font (typeface and size), each character corresponds to a single glyph but this is not always the case, especially in a language with a large alphabet where one character may correspond to several glyphs or several characters to one glyph (a character encoding). The term is usually used in particular reference to outline fonts.
In graphonomics, the term glyph is used for a non-character, i.e., either a sub-character or multi-character pattern.