Parallel

Parallel is one of the 117 mis-spellings listed as 'Common difficulties' in the section on 'Spelling' within 'Writing' in UEfAP.

Note that this word and its derivatives are spelled with 2 'l's in the middle, and only one 'l' at the end: parallel. The same is true of parallelism. The verb 'to parallel' has participles paralleling and paralleled (with unparalleled meaning 'matchless').