The diaeresis (UK /daɪˈɪrᵻsᵻs/, US /daɪˈɛrᵻsᵻs/ dy-ERR-i-sis; plural: diaereses), also spelled diæresis or dieresis and also known as the trema or the umlaut, is a diacritical mark that consists of two dots ( ¨ ) placed over a letter, usually a vowel. When that letter is an i or a j, the diacritic replaces the tittle: ï.