Diaeresis vs. Umlaut — What's the Difference?

Diaeresis, often seen in English and other languages, is a diacritical mark consisting of two dots (¨) placed over a vowel to signal that the vowel should be pronounced apart from the vowel before it, ensuring that each vowel maintains its own syllable. On the other hand, the umlaut, also marked by two dots, is used primarily in German and some other languages to indicate a change in the sound of the vowels a, o, and u.

In terms of usage, a diaeresis is used in words where vowels might otherwise be mistaken for a diphthong or a silent vowel, as in "coöperate" where it prevents the blending of the two 'o's into a single sound. Whereas, an umlaut changes the pronunciation of the vowels: "a" becomes the fronted sound [æ], "o" is changed to [ø], and "u" to [y].

The diaeresis is also used in loanwords and names where pronunciation may not be intuitive for English speakers, such as in the name "Brontë." Conversely, the umlaut has grammatical implications in German, affecting word forms and pluralization, such as changing "Mutter" (mother) to "Mütter" (mothers).

While the visual representation of diaeresis and umlaut is the same, their functions are quite distinct. The diaeresis prevents the merging of vowels into a single syllable, maintaining clarity in pronunciation, whereas the umlaut fundamentally alters the vowel sound to convey different lexical or grammatical meanings.

While the diaeresis appears primarily in literary or formal contexts to aid pronunciation, the umlaut is integral to everyday language use in German, affecting the basic sounds of words and thereby their meanings and roles within sentences.