Diaerese

Diaeresis, also called diæresis or diaeresis, is a diacritic consisting of two dots placed over a vowel. From the Greek diairesis meaning division, it signals that two adjacent vowels form separate syllables rather than a single vowel sound.

In languages that use it, the diaeresis marks hiatus or separation of syllables. In French, it appears over vowels to indicate that the vowels belong to distinct syllables, as in Noël or naïve, preventing the second vowel from forming a glide with the first. In English, diaeresis is used in a few loanwords or proper names (naïve, coöperate, reëvaluate), though it is increasingly uncommon. In Spanish, the diaeresis on the u (ü) shows that the u is pronounced in sequences that would otherwise be silent (vergüenza, pingüino).

The term also covers the Dutch twee-dots on vowels used to indicate syllable separation in certain words, though in Dutch the mark is more commonly called trema. The diaeresis is distinct from the umlaut, which marks vowel quality changes in Germanic languages.

In typography, the two dots can appear over any vowel letter, producing characters such as ï, ö, or ü, and are encoded in Unicode as the diaeresis U+00A8 or as a combining diaeresis U+0308.