Dictionary.com Word FAQs
How do I know whether to double a final letter when adding a suffix?
We double the final consonant of a word before we add -ed, -er, -est, -ing, -able and -y to show that the vowel has a short sound. When a one-syllable word ends in a consonant preceded by one vowel, double the final consonant before adding a suffix which begins with a vowel. This is also called the 1-1-1 rule: one syllable, one consonant, one vowel. Examples are: bag, baggage; bat, batted, batting, batter. (The two notable exceptions to this are bus, gas.) When a multi-syllable word ends in a consonant preceded by one vowel, and the accent is on the last syllable, double the final consonant. Examples: confer, conferring; control, controlled; begin, beginning. In words of more than one syllable ending in a consonant, especially l, British English generally doubles the final consonant, and American English generally does not, though it is certainly acceptable. Examples: canceled, cancelled; labeled, labelled. Remember that only some letters are doubled: b, d, g, l, m, n, p, r, t.