RULE: 4
The final consonant is preceded by a single vowel:
bar → barred. When there is more than one vowel before the final
consonant, the consonant is not doubled:
fail → failed. When the final consonant is preceded by another consonant,
the consonant is not doubled:
bark → barking.
The word has only one syllable or has the stress on the last syllable:
fit → fitted,
commit → committed,
prefer → preferred. For words with more than one syllable where the stress does not fall on the last syllable, the final consonant
is not doubled:
benefit → benefited, offer → offered. The suffix begins with a vowel:
prefer → preferred. But if the syllable stress changes because of the addition of the
suffix, then the consonant is not doubled:
prefer → preference. Final consonants are also not doubled if the suffix
begins with a consonant:
prefer → preferment. If the word ends in l or p, then the consonant is usually doubled in Canadian spelling:
travel → travelled; worship → worshipped. Note: American spelling does not follow this rule.
Source:
www.writing.utoronto.ca/advice/style-and-editing/spelling
