trema

A trema (from the Greek τρῆμα trêma; plural tremas or tremata) is a diacritic consisting of two dots ( ¨ ) placed over a letter, most commonly a vowel. (When that letter is an i, the diacritic replaces the tittle: ï.) The trema is usually used to denote one of two distinct phonological phenomena: diaeresis (pronounced /daɪˈɛrɨsɨs/ dy-ERR-ə-səs), in which the trema is used to show that a vowel letter is not part of a digraph or diphthong; and umlaut (pronounced /ˈʊmlaʊt/ OOM-lowt), in which the trema illustrates a sound shift.