Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
CONSONANTS 2, DOUBLING OF
Here are some useful generalizations about this spelling issue:
Words that end in a single vowel plus a single consonant usually double the final consonant before adding a suffix that begins with a vowel: stop becomes stopped, stopping, stopper, and unstoppable. Thus snip becomes snipper, but snipe becomes sniper.
Most words that end in two consonants do not ordinarily double the final consonant before a suffix: print becomes printed, printing, and printer.
If the suffix begins with a consonant instead of a vowel, the final consonant of the base word stays single: ship becomes shipment and clap becomes claptrap.
Words of two and more syllables that are stressed on the final syllable normally double the final consonant before adding a suffix: infer becomes inferred and inferring.
But two-syllable words stressed on the final syllable do not double the final consonant when the suffix begins with a consonant: regret becomes regretting but regretful.
And words stressed on the final syllable but ending with two consonants or with a vowel do not double the consonant: predict becomes predicting and predicted; reduce becomes reducer and reduced.
In words of more than one syllable ending in a consonant, especially -l, the English generally (but not always) double the final consonant, and Americans generally do not, although American dictionaries frequently report divided usage. Here are some examples:
