Diacritic

–a glyph added to a letter of the alphabet or to a basic glyph.

There are quite a few of them in various languages as listed on this Wikipedia page and they all have the effect of indicating a different pronunciation to a letter without one.

English doesn’t have any, one of the few languages which doesn’t except for words borrowed from French; neither does Dutch although it doubles a vowel to show when it’s long. Maori could do that too, but uses macrons over vowels instead.

Sometimes fake diacritics are added by rock bands to their names, and in Kapiti a decade ago, Mayor Rowan decided to add a fake macron over the ‘a’ which is still used despite criticism by both Maori and non-Maori.