From Hull AWE
Successful is one of the 117 mis-spellings listed as 'Common difficulties' in the section on 'Spelling' within 'Writing' in UEfAP. It may be helpful to look at its structure:
- The noun success has two '-c-'s, to mark the '-k- + '-s-' sound with which it is pronounced (IPA: /sək ˈsɛs/, and two '-s-'s to mark the stress with which it is pronounced - on the second syllable.
- The adjective successful is composed of success + the adjectival suffix -ful, which has only one '-l'.
- The adverb successfully adds the adverbial suffix -ly to successful in the usual way, thus ending in -lly, with one '-l' from 'successful';' and another from '-ly'.
- This makes successfully one of a small number of English words with three double letters.