Diæresis

The diæresis (See Accents) placed over a vowel denotes in general that that vowel forms a syllable, and does not constitute part of one with another vowel preceding or following it. Thus aërial is pronounced a-e-rial. So preëminent, and similar words, where the two vowels are part of two different syllables, are sometimes distinguished by the diæresis; but the usual plan is to insert a hyphen between the two vowels as in co-operate.