There is no rule or consistent pattern governing the formation of adjectives ending in -ic and -ical. Some -ic words are preferred over their -ical counterparts (e.g., ironic over ironical), and some –ical words are preferred over their -ic counterparts (e.g., metaphorical over metaphoric, alphabetical over alphabetic). Still other -ic and -ical pairs have differentiated over time and now have different meanings. Historic and historical, for instance, have different definitions, as do periodic and periodical, classic and classical, and economic and economical.
When you’re in doubt about which form is preferred or whether an -ic/-ical word pair has differentiated, the only way to know for sure is to check a dictionary or other reference source.
For more information on specific –ic/-ical word pairs, click below.