what are two dots over a letter called?
The diaeresis (/daɪˈɛriːsɪs/ dy-ERR-ee-sis; also known as the tréma) and the umlaut are two different homoglyphic diacritical marks. They both consist of two dots ( ¨ ) placed over a letter, usually a vowel. ... The diaeresis and the umlaut are diacritics marking two distinct phonological phenomena.