“ Letters are divided into VOWELS and CONSONANTS.
The Vowels are those letters which can be perfectly sounded without the
aid of any other letter. The vowels are a, e, i, o, u, w, and y.
The vowel sounds of w and y are the same as those of u and i. A, e, and o
are always vowels. I, u, w, and y are sometimes consonants.
A Diphthong is the union of two vowels in one sound. When both vowels are
sounded, the diphthong is called PROPER, because then it is really a
DIPHTHONG, or double sound; that is, the sounds of the vowels unite; as,
oi in oil; ou in sound. ”