Combining character

[1] In Unicode, the main block of combining diacritics for European languages and the International Phonetic Alphabet is U+0300–U+036F.

Combining diacritical marks are also present in many other blocks of Unicode characters.

Combining characters are assigned the Unicode major category "M" ("Mark").

For example, U+0364 is an e written above the preceding letter, to be used for (Early) New High German umlaut notation, such as uͤ for Modern German ü. OpenType has the ccmp "feature tag" to define glyphs that are compositions or decompositions involving combining characters, the mark tag to define the positioning of combining characters onto base glyph, and mkmk for the positionings of combining characters onto each other.

It is typically very challenging for most software to render, so the combining marks are often reduced or completely stripped off.