Complement or compliment: which is which?

Complement, spelled with an e in the middle, means something that completes or goes well with something else: the wine complements the fish, a full complement of staff, complementary colors. Compliment, with an i, means praise or a courteous expression of admiration: she complimented his cooking, a backhanded compliment. Both function as noun and verb. The reliable mnemonic is that complement contains the word complete, which is exactly what it means, and that a compliment is something I like.

The adjective forms cause a related and frequent error. Complementary means completing or matching, as in complementary angles that sum to 90 degrees or complementary skills on a team. Complimentary has two senses: expressing praise, and given free of charge, as in a complimentary breakfast, which derives from the idea of a courtesy extended. That second sense is where most of the confusion lands, since a hotel offering complementary breakfast would be claiming the meal completes something rather than that it is free.

Both descend from the same Latin root complere, meaning to fill up or complete, which is why they look alike and why the split feels arbitrary. Compliment reached English through Italian and French with the sense of a courtesy that fulfills social obligation, and the spellings diverged only in the seventeenth century. Two adjacent pairs worth checking with the same care because spellcheck accepts either: principal and principle, where the school principal is your pal and a principle is a rule, and stationary and stationery, where stationery with an e includes envelopes.