diëresis

Diéresis, also called diaeresis or dieresis in some spellings, is a diacritic consisting of two dots placed over a vowel. In typography and linguistics it signals that adjacent vowels form separate syllables (hiatus) or, in certain languages, that the vowel should be pronounced distinctly from the surrounding vowels. The term is used in several languages with varying functions.

In Spanish, diéresis refers specifically to the diaeresis on the letter u (the letter ü). It indicates that the u is pronounced in sequences such as güe and güi, where otherwise the u would be silent or merged with the preceding consonant. Examples include vergüenza and pingüino. The mark is also used in some proper names and loanwords to clarify pronunciation.

In other languages, the diacritic may serve different purposes. In French, a closely related mark called the trema or diaeresis indicates that two adjacent vowels are to be pronounced in separate syllables, as in Noë l. In English and other languages, the diaeresis is used less commonly today but appears in words such as naïve, Noël, and Zoë to show that a vowel begins a new syllable or to prevent the vowels from combining into a diphthong. Historical usage includes coöperate or reëvaluate, though such spellings are now rare.

The two-dot mark is distinct from the German umlaut, which uses the same diacritic to alter a vowel’s quality rather than to indicate hiatus. In typography, the symbol’s function depends on the language and historical orthography.