The Complete Guide to English Spelling Rules

(^) Occasionally there will be an accent shift when a suffix is added to a word. In these cases, the spelling
rule must be followed. The words that fit the rule will double the consonant, but those that do not fit the
rule will not:
(^) When adding in or un as prefixes, use only one n unless the word starts with an n:
(^) When adding mis or dis as a prefix, use only one s unless the word starts with an s:
(^) When adding ly to a word that ends in ic, use ally (two l’s):
(^) As a rule of thumb, when in doubt or when the dictionary offers a choice, do not double the consonant.