Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
VOWELS, THE AMERICAN ENGLISH
The English vowel letters are a, e, i, o, u, and sometimes y, but the vowel sounds are more numerous. They are characterized by the continuous sounds they make as the main part of any syllable. Certain vowel sounds appear only in one or more regional dialects, but the following, represented here with the pronunciations used in this book, are the vowel sounds most usually encountered in Standard American English:
