travelled

1325–75; Middle English (north and Scots), orig. the same word as travail (by shift “to toil, labor” > “to make a laborious journey”)

The word travel has come to exemplify a common spelling quandary: to double or not to double the final consonant of a verb before adding the ending that forms the past tense ( –ed ) or the ending that forms the present-participle ( –ing. ) We see it done both ways—sometimes with the same word ( travel, traveled, traveling; travel, travelled, travelling ). As readers, we accept these variations without even thinking about them. But as writers, we need to know just when we should double that final consonant and when we should not. Because American practice differs slightly from British practice, there is no one answer. But there are well-established conventions.
In American writing, when you have a one-syllable verb that ends with a single vowel followed by a single consonant, and you want to add a regular inflectional ending that begins with a vowel, you double that final consonant before adding -ed or -ing : stop, stopped, stopping; flag, flagged, flagging. This principle also holds for verbs of more than one syllable if the final syllable is stressed: permit, permitted, permitting; refer, referred, referring. If that syllable is not stressed, there is no doubling of the final consonant: gallop, galloped, galloping; travel, traveled, traveling.
British spelling conventions are similar. They deviate from American practices only when the verb ends with a single vowel followed by an l . In that case, no matter the stress pattern, the final l gets doubled. Thus British writing has repel, repelled, repelling (as would American writing, since the final syllable is stressed). But it also has travel, travelled, travelling and cancel, cancelled, cancelling, since in the context of British writing the verb’s final l, not its stress pattern, is the determining factor. Verbs ending in other consonants have the same doubling patterns that they would have in American writing. An outlier on both sides of the Atlantic is the small group of verbs ending in -ic and one lonely -ac verb. They require an added k before inflectional endings in order to retain the appropriate “hard” sound of the letter c : panic, panicked, panicking; frolic, frolicked, frolicking; shellac, shellacked, shellacking. Canadians, of course, are free to use either British or American spellings.

late Middle English word dating back to 1375–1425; see origin at travel, -ed²