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The letters f, s, and l at the end of words of one syllable and coming immediately after a single vowel are generally doubled: eg, cliff, gross, stroll). Other consonants are rarely doubled: eg, job, tip, cat).
The letters f and s (but not l) at the end of words of more than one syllable and coming immediately after single vowels are frequently doubled: eg, sheriff, surpass).
The letters f, s, and l at the end of words and coming immediately after double vowels (or vowels that sound like double vowels; ie, diphthongs) are rarely doubled: eg, sheaf, stool; exception: guess)
Derivatives of words that are formed by prefixing or adding one or more syllables to words ending in double consonants commonly retain both consonants (but the words all, well, full, and mass are exceptions to this rule, and usually omit one of the consonants): eg, foretell, recall; stillness, but almost, already, welcome, welfare, faithful, useful, Christmas.
When ly is added to a word ending with a single l, both l's are retained: eg, carefully, finally, cheerfully.
Source: Eldridge's Business Speller.
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