Precomposed character

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A precomposed character (alternatively decomposable character) is a Unicode entity that can be decomposed into an equivalent string of several other characters. Typically, a precomposed character is decomposed into the main character and a combining diacritical mark.

On most computer systems with incomplete Unicode support, the precomposed characters are easier to handle, and also they look better on displays and in print.

For example, the two strings

ḱṷṓn (U+006B U+0301 U+0075 U+032D U+006F U+0304 U+0301 U+006E) and

ḱṷṓn (U+1E31 U+1E77 U+1E53 U+006E)

are equivalent and should render identically. In practice, however, most rendering engines still have difficulties with displaying the decomposed string.

In principle, most Chinese characters as encoded by Han unification and similar schemes may be considered precomposed characters, since they can be reduced (decomposed) to their constituent strokes. Similarly, ligatures are precompositions of their constituent graphemes.

OpenType has the ccmp "feature tag" to define glyphs that are compositions or decompositions involving combining characters.