long ōō as in room, soon, mood, etc.
short ŏŏ, as in good, took, wood, etc.
Ow has a sound of its own, as in how, crowd, allow, etc.; and ou sometimes has the same sound, as in loud, rout, bough, etc.
(Ow and ou are also sometimes sounded like long o, as in own, crow, pour, etc., and sometimes have still other sounds, as ou in bought).
Oi and oy have a distinct sound of their own, as in oil, toil, oyster, void, boy, employ, etc.
Ow and oi are called proper diphthongs, as the two vowels combine to produce a sound different from either, while such combinations as ei, ea, ai, etc., are called improper diphthongs (or digraphs), because they have the sound of one or other of the simple vowels.
3. In the preceding paragraphs we have given all the distinct vowel sounds of the language, though many of them are slightly modified in certain combinations. But in many cases one vowel will be given the sound of another vowel, and two or more vowels will combine with a variety of sounds. These irregularities occur chiefly in a few hundred common words, and cause the main difficulties of spelling the English language. The following are the leading substitutes:
ew with the sound of u long, as in few, chew, etc. (perhaps this may be considered a proper diphthong);
e (ê, é) with the sound of a long, as in fête, abbé, and all foreign words written with an accent, especially French words;
i with the sound of e long, as in machine, and nearly all French and other foreign words;