Grammar, a double vowel, or the mixture of two vowels pronounced together so as to make one syllable.

The Latins pronounced the two vowels in their diphthong ae or oe, or oe or oe, much as we do, only that the one was heard much weaker than the other, though the division was made with all the delicacy imaginable. Diphthongs with reference to sight, are distinguished from those with reference to sound. In the former, either the particular sound of each vowel is heard in the pronunciation; or the sound of one of them is drowned; or, lastly, a new sound, different from either, results from both; but the first of these only are real diphthongs, as being such both to the eye and ear. Diphthongs with regard to the ear are either formed of two vowels meeting in the same syllable, or of two vowels whose sounds are severally heard; or of three vowels in the same syllable, which only afford two sounds in the pronunciation.

English diphthongs, with regard to the eye and ear, are ai, au, ea, ee, ei, oo, ou. Improper English diphthongs, with regard to the eye only, are aa, ae, eo, eu, ie, ei, oe, ue, ui.