- A vowel ligature - A coalition or union of two vowel sounds pronounced in
one syllable; as, ou in out, oi in noise; -- called a proper diphthong. - A vowel digraph; a union of two vowels in the same
syllable, only one of them being sounded; as, ai in rain, eo in people;
-- called an improper diphthong. - To form or pronounce as a diphthong; diphthongize.