Diphthongs and Tripthongs
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.
When only one of the vowels is sounded, the diphthong is called IMPROPER, because then, as one of the vowels is silent, it is not properly a DIPHTHONG, though it takes that name; as, oa in boat, ui in suit, where a and i are silent.
The following diphthongs are in common use, viz.: oi, oy, ou, ow, ae, ai, au, aw, ay, ea, ei, eo, eu, ew, ey, ia, ie, oa, oe, ua, ue, ui; as in toil, boy, round, plow, seal, coal, head, sail, say, aught, yeoman. Of these, oi, oy, ou, and ow are generally proper diphthongs; though sometimes ou and ow are improper, as in famous, where o is silent, and in slow, where w is silent.
A Triphthong is the union of three vowels in one syllable; as, eau in beau, iew in view. The triphthong is properly a union of letters, not sounds.