diaresis

The diaeresis, sometimes spelled diaresis, is a diacritic consisting of two dots placed over a vowel. In languages that use it, the diaeresis signals that two adjacent vowels are to be pronounced separately, forming two syllables rather than a single digraph or diphthong (a hiatus).

In English, the diaeresis appears only in a small set of words or in older typography, mainly to indicate that adjacent vowels are to be pronounced separately. Examples include naïve, coöperate, reëntry. Modern practice often renders these words without the two dots, or uses ligatureless spellings.

Other languages retain more regular usage. In French, the mark is called the trema (tréma) and is used on vowels such as ï, ë, ü to show that the vowels belong to separate syllables, as in Noël. In Spanish and Catalan, the diaeresis (on ü or ï) indicates that the vowel should be pronounced distinctly from the preceding vowel, as in pingüino or vergüenza; in German the two-dot mark is more commonly called an umlaut, a related diacritic that alters vowel quality rather than simply signaling hiatus.

Origins lie in Greek diairesis meaning division or separation. In typography, the two dots can be placed over a, e, i, o or u, and in Unicode are encoded as the diaeresis diacritic either as a separate character (¨) or as a diacritic combining mark.