Complement and compliment are pronounced almost identically, which is exactly why they get swapped so often — but they mean completely different things. Complement, spelled with an e, means to complete, enhance, or pair well with something. Compliment, spelled with an i, means to praise someone, or an expression of admiration. The fastest test: if you can replace the word with praise, use compliment; if you mean completes or goes well with, use complement.
Keep the two words apart by their spelling and their core idea:
Both words trace back to the same Latin verb, complēre, "to fill up or complete," and in the 1600s writers used the two spellings more or less interchangeably. English later split them: complement kept the idea of completing, while compliment drifted toward the social sense of a courteous, "completing" remark. Their shared origin is why they still sound alike today.
Complement works as both a verb and a noun, and the through-line is always completion.
As a verb, to complement something is to complete it, improve it, or go well with it: Her calm complements his energy. As a noun, a complement is the thing that does the completing: A silk scarf is the perfect complement to the dress. It also means a full quantity or set — the ship sailed with its full complement of crew — and it carries technical senses in grammar (a subject or object complement), mathematics (complementary angles add up to 90°), and biology (the complement system in immunology).
Notice that a complement need not be identical to what it completes; often it is different but fitting, like two puzzle pieces. Skills, colors, flavors, and personalities can all complement one another.
Compliment is also a verb and a noun, and its core idea is praise.
As a verb, to compliment someone is to tell them something admiring: He complimented her on the presentation. As a noun, a compliment is that admiring remark: "Nice work" is a genuine compliment. In the plural, compliments can mean formal greetings or regards — please give my compliments to the chef, or with compliments on a gift card. And when something is described as complimentary, it is either praising (a complimentary review) or free of charge (a complimentary breakfast), because it is offered as a courtesy.
The adjectives cause even more trouble than the base words, so it is worth pinning them down:
The famous trap is the hotel breakfast. A complimentary breakfast is free — think of it as a courtesy, a "compliment" from the hotel. A complementary breakfast would be one that somehow completes another meal, which is almost never what a menu means.
Use complement when something completes or matches:
Use compliment when someone is praising:
And the free-versus-matching split:
"A full complement." This fixed phrase means the complete number required, never praise: the team now has its full complement of coaches.
The "scarf" ambiguity. Occasionally either word is technically possible, and only your meaning decides. The scarf complements her outfit means it completes the look; the scarf compliments her outfit would mean the scarf is somehow praising it — poetic, but almost certainly not what you intend. When in doubt, ask whether an act of praise is really happening.
Compliments as regards. On a gift, "with compliments" and a compliments slip use the praise/courtesy word, spelled with an i.
Technical complements. In grammar, a complement completes the meaning of a verb or subject (in She is a doctor, "a doctor" is a subject complement). In geometry, two complementary angles total 90° (two supplementary angles total 180°). All spelled with an e.
Two tricks cover almost every case:
For the adjectives, remember that complimentary and free both feel like a gift: a complimentary upgrade costs you nothing, just as a compliment costs nothing to give.
No — and that is worth stating plainly, because many confusable pairs really do split across the Atlantic. Complement and compliment keep exactly the same spellings and meanings in American, British, Australian, and every other standard variety of English. The only variation is minor spelling elsewhere in the sentence (complementary colours in the UK, complementary colors in the US), never in these two words themselves. This is purely a same-sound, different-spelling trap, not a dialect issue — so the rules above apply wherever you write.
It's a complimentary breakfast, spelled with an i, because the meal is free. Complimentary means given as a courtesy at no charge — the hotel is effectively paying you a small compliment. Complementary, with an e, would mean the breakfast completes something else, which is not what hotels advertise.
Usually complement each other, with an e, meaning two things combine well to form a balanced whole — as in colleagues whose skills complement each other. Compliment each other, with an i, is also correct English, but it specifically means they exchange praise. Choose based on whether you mean completing or praising.
Link complement to complete — both start with comple- and both mean finishing or filling out a whole. For compliment, remember the i and the phrase 'I love a compliment,' since a compliment is praise. If you can swap the word for praise, use compliment; if you mean completes, use complement.
The standard, most idiomatic choice is to compliment someone on something: she complimented him on his cooking. Compliment someone for something is also acceptable, especially when praising an action or achievement. You can also pay someone a compliment or give a compliment. All use the i spelling, since praise is involved.
Complementary, with an e, means combining well to complete a whole — complementary colors or complementary skills. Complimentary, with an i, means either praising (a complimentary review) or free of charge (a complimentary drink). If nothing is actually being completed, you almost always want the i spelling.
