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The easiest rule to remember is: when a word ends with a consonant, preceded by a single vowel, then that consonant is doubled in the past simple tense. But in American English if a word ends with a single "l" it is left alone. BrEng travelled AmEng traveled
@Mari-Lou: when you answered, you should have said there were numerous exceptions. For another example, compelled. There is a more complete rule which says that for multi-syllable words in American English, you double the final letter only if the last syllable ends with a consonant, contains a single-letter short vowel, and is accented. But again, it has exceptions.