(In American English, this is spelled dieresis.)
Diaeresis (pronounced 'die-ER-iss-iss', IPA: /daɪ'ɛrɪsɪs/ - the plural is diaerises, 'die-ER-iss-iz' or 'die-er-iss-EEZ', IPA: /daɪ'ɛrɪsɪz/ or IPA: /daɪɛrɪ'siːz/) is a word with several different, but related, meanings, which you are unlikely to find outside academic English.
- Most commonly, it labels the mark (¨) placed over a letter, as in ï or ö, used to show that two vowels are to be treated as separate sounds, not as a single diphthong, as in naïve, Zoë, and coöperate, or to show that a vowel is to be pronounced and not left silent, as in Brontë ('BRONT-y', NOT 'bront'). Many words that used invariably to be written with a diaeresis are now to be seen, even from the most reputable publishers, without it: 'naive', 'co-operate' (even cooperate, which AWE does not recommend, as being awkward to read), 'Zoe', and 'Bronte' are all to be seen.
- The ¨ mark is also called an umlaut or a trema.
- More properly, diaeresis is the name of the phenomenon which the mark shows: the pronunciation of two vowel sounds separately, rather than as one.
- A diaeresis in the sense of pronunciation as separate vowels does not need to be marked with a diaeresis in the sense of a printed or written mark. The mythological name 'Orpheus', for example, has three syllables, not two: 'or-phy-oos', not 'or-phoos'. In English, it is never written with the ¨ mark, although it is always pronounced in the trisyllabic way.