Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
SPELLING OF UNSTRESSED VOWELS
Spoken English is full of unstressed vowels, and we spell them with almost every vowel letter and combination in the alphabet. Consider the spellings of words such as the adjectives separate, desperate, or indolent. Consider too words that might end in either -ance or -ence, -ibility or -ability. For most bad spellers, unstressed vowels are the greatest single cause of error; therefore, whenever you are unsure, consult your dictionary. See also -ABLE.
