[2][b] The diaeresis diacritic indicates that two adjoining letters that would normally form a digraph and be pronounced as one sound, are instead to be read as separate vowels in two syllables.
[4] In English language texts it is perhaps most familiar in the loan words naïve, Noël and Chloë, and is also used officially in the name of the island Teän and of Coös County.
[5] The word trema (French: tréma), used in linguistics and also classical scholarship, is from the Greek trē̂ma (τρῆμα) and means a "perforation", "orifice", or "pip" (as on dice),[6] thus describing the form of the diacritic rather than its function.
In Greek, two dots, called a trema, were used in the Hellenistic period on the letters ι and υ, most often at the beginning of a word, as in ϊδων, ϋιος, and ϋβριν, to separate them from a preceding vowel.
[7][8] The diaeresis was borrowed for this purpose in several languages of western and southern Europe, among them Occitan, Catalan, French, Dutch, Welsh, and (rarely) English.
To indicate exceptions to this rule (hiatus), a diaeresis mark is placed on the second vowel: without this the words raïm [rəˈim] ("grape") and diürn [diˈurn] ("diurnal") would be read *[ˈrajm] and *[ˈdiwrn], respectively.
Examples include the given names Chloë and Zoë, which otherwise might be pronounced with a silent e. To discourage a similar mispronunciation, the mark is also used in the surname Brontë.
Ÿ occurs in French as a variant of ï in a few proper nouns, as in the name of the Parisian suburb of L'Haÿ-les-Roses [la.i le ʁoz] and in the surname of the house of Croÿ [kʁu.i].
In Galician, diaeresis is employed to indicate hiatus in the first and second persons of the plural of the imperfect tense of verbs ended in -aer, -oer, -aír and -oír (saïamos, caïades).
In addition, identically to Spanish, the diaeresis is used to differentiate the syllables güe [ɡʷe] an güi [ɡʷi] from gue [ɡe] and gui [ɡi].
[13] In German, in addition to the pervasive use of umlaut diacritics with vowels, diaeresis above e occurs in a few proper names, such as Ferdinand Piëch and Bernhard Hoëcker.
In Modern Greek, αϊ and οϊ represent the diphthongs /ai̯/ and /oi̯/, and εϊ the disyllabic sequence /e.i/, whereas αι, οι, and ει transcribe the simple vowels /e/, /i/, and /i/.