A modifier letter, in the

Unicode Standard Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, whic ...

, is a letter or symbol typically written next to another letter that it modifies in some way. They generally function like

diacritics A diacritic (also diacritical mark, diacritical point, diacritical sign, or accent) is a glyph added to a letter or to a basic glyph. The term derives from the Ancient Greek (, "distinguishing"), from (, "to distinguish"). The word ''diacriti ...

, changing the sound-values of the letter it is next to (usually the letter preceding it but sometimes the following letter instead). Like combining marks, they are often used in technical phonetic transcriptional systems to make phonetic distinctions.7.8 Modifier Letters, p.321
''The Unicode Standard, Version 13.0'', 2020

References

Unicode {{Digital-typography-stub