Diaeresis

Diaeresis, also called dieresis, is a diacritical mark consisting of two dots placed over a letter. It signals that the two adjacent vowels are to be pronounced separately rather than as a single sound or digraph.

Usage varies by language. In Greek orthography the diaeresis marks hiatus, indicating that two vowels are pronounced in separate syllables rather than as a diphthong. In French and several other languages, the diaeresis (as over i, e, or u) shows that the vowel should be pronounced distinctly from the preceding vowel, as in naïf, naïve, Noël, and Haïti. In Dutch and older English texts, it has been used to clarify pronunciation in words like coöperatie or coöperate, though it is less common today. In Spanish, diaeresis (diéresis) also appears on the letter ü to indicate that the u is pronounced in sequences such as güe or güi, rather than being silent.

Distinction from umlaut is important. In German and related languages, the two dots denote an umlaut, a vowel-quality change rather than a separation of syllables. In many contexts the marks look identical, but they serve different phonological purposes and belong to different orthographic traditions.

In typography and encoding, the diaeresis is represented by the two-dot diacritic over a letter, as in ï, ë, ö, and ü. Unicode provides the diaeresis as a standalone diacritic (U+00A8) and as combining marks (U+0308), allowing its use over any letter in digital text.