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 ADec 22 '13 at 09:48
@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.
– Peter Shor Dec 22 '13 at 11:32
So for OP's 'What is the rule' read 'Is there anything we could even term a rule of thumb . . .'.
– Edwin AshworthDec 22 '13 at 12:09
@PeterShor I gave an easy rule and I gave him the link to a more complete answer. Anyway, I thought comments were not considered answers.
– Mari-Lou ADec 22 '13 at 12:20