When adding certain endings such as -ed, -ing, -er, and -est to words, we sometimes double consonants (doubling consonants). These endings represent the past tense, progressive tense, comparative, and superlative respectively. But consonants aren’t always doubled when adding these suffixes, so it’s important to learn the rules of spelling words with double consonants.
We often double the final consonant of a word (b, d, g, l, m, n, p, r, t) when a suffix beginning with a vowel is added (-ed, -er, -est, -ing):
hop + -ed → hopped
slim + -ing → slimming
red + -ish → reddish
thin + -er → thinner
rub + -ed → rubbed
travel+ -er → traveller
sit + -ing → sitting
wet + -er → wetter
When we add a suffix to a word with more than one syllable, we double the consonant only when the word ends in a stressed syllable (the stressed syllable of the base form is in bold):
admit + -ing → admitting
prefer + -ed → preferred
forget + -ing → forgetting
transmit + -ed → transmitted
occur + -ence → occurrence
upset + -ing → upsetting
Compare, however, visit or enter where the spoken stress is on the first syllable:
visit → visiting
enter → entered
Note too that in each case the vowel before the last consonant is a short vowel.
We don’t double the final consonant before a suffix:
– if the word ends in two written consonants, e.g. export = exported, find = finding, insist =insisted, lift = lifted, persist = persistence
– if there are two written vowels together in the word, e.g. meeting, rained, weaken, trainer, repeated.
Irregular forms and exceptions
Some monosyllabic words ending in -s are irregular. We normally do not double the -s, although some doubled forms will be seen. For example: busses and buses; gasses and gases. (Bussesand gasses are not common.)
Some words, several of them ending in l, with more than two syllables, have a double consonant even though the last syllable is not stressed; for example, labelling, traveller, equalled, handicapped, programmed.
In American English the single consonant spelling is usually more common: labeling, traveler.