An improper diphthong is a diphthong in which only one of the vowels is sounded; as, oa in loaf, eo in people.

ie and ue, derived from the Latin e and o respectively, form indissoluble diphthongs.

Oy is a proper diphthong, equivalent in sound to oi; as in joy, toy, oyster.

as obscure e diphthongs beginning with triphth.

The long sound of u is yu; hence Walker calls the letter, when thus sounded, a "semi-consonant diphthong.

The long sound of u is yu; hence Walker calls the letter, when thus sounded, a "semi-consonant diphthong.

After his explanation of these mysteries, he says, "The seven radical sounds with their vanishes, which have been described, include, as far as I can perceive, all the elementary diphthongs of the English language."Ib., p. 60.

[Footnote 2: The Opt. has an extra mood-diphthong, ει, before ending.]