In two-syllable verbs the final consonant is doubled when the last syllable contains a single vowel letter followed by a single consonant letter and is stressed: pre'fer/preferred, re'fer/referred. Compare: 'benefit/benefited, 'differ/differed and 'profit/profited which are stressed on their first syllables and which therefore do not double their final consonants. In American English labeled, quarreled, signaled and traveled follow the rule. In British English labelled, quarrelled, signalled and travelled are exceptions to the rule.