Complimentary vs complementary: free, flattering or a good match

Complementary, with an e, means completing something or going well with it. Complimentary, with an i, means given free as a courtesy, or praising. One hotel room can show both: the breakfast is complimentary, included at no extra charge, and the blue walls and orange cushions are complementary colors, each making the other look stronger. The memory trick: a complement completes, and a compliment is something I like to get, whether it's praise or a free coffee.

The praising sense is the one people forget. A complimentary review is a favorable one, and a manager who was complimentary about your report praised it. Both senses come from compliment, an admiring remark, so both take the i. The two words sound exactly alike, which means the mistake lives only in writing, and it tends to show up where many people will see it: menus, hotel pages, event programs, conference badges and book proposals.

Complementary vs complimentary at a glance

What complementary means

Merriam-Webster defines complementary as serving to complete something, and as “mutually supplying each other's lack.” Two things are complementary when each has what the other is missing: a writer with ideas and an editor with structure, a sweet glaze on a salty ham, a quiet partner and a talkative one. The noun complement names the thing that completes (a perfect complement to the meal), and the verb means to complete or enhance (the photographs complement the essay).

Several fields use the word in a precise sense:

• Color. Complementary colors are pairs that cancel each other out when mixed, leaving a neutral gray, white or black. On the traditional painter's wheel the pairs are red and green, yellow and purple, and blue and orange. In light they are red and cyan, green and magenta, and blue and yellow. Placed side by side, complements make a strong contrast, which is useful on a cover or a slide.
• Geometry. Complementary angles add up to 90 degrees, so a 30-degree angle and a 60-degree angle are complements. Supplementary angles add up to 180. One way to remember it: C for corner, S for straight line.
• Biology. The two strands of DNA are complementary: adenine pairs with thymine and guanine with cytosine, so one strand determines the other. In RNA, adenine pairs with uracil. Complementary DNA, or cDNA, is built from messenger RNA by the enzyme reverse transcriptase.
• Medicine. The US National Center for Complementary and Integrative Health calls a non-mainstream approach complementary when it is used together with conventional medicine, and alternative when it is used in place of it.
• Publishing. In a nonfiction proposal, some writers split their comp titles into competitive books, which a reader might buy instead of yours, and complementary ones, which they would buy alongside it. The book proposal template has a table for them.
• Grammar. A complement is the word or phrase that completes a predicate. In The soup tastes salty, salty is a subject complement; in The board named her chair, chair is an object complement.

Complement also means the full number of something: a full complement of staff, or, on a ship, the whole crew. That sense keeps the e, because a full complement is the number that makes the set complete.

What complimentary means

Merriam-Webster gives complimentary two meanings:

• Expressing praise, or favorable: She was complimentary about the second draft. The novel received complimentary reviews. The opposite is uncomplimentary.
• Free, offered as a courtesy: complimentary tickets, a complimentary shuttle, complimentary copies for reviewers.

The noun compliment covers the same ground. A compliment is an admiring remark, and in the plural, compliments are best wishes, as in compliments of the season or my compliments to the chef. Merriam-Webster also lists compliments of for naming the giver of something free (dessert, compliments of the house) and, ironically, the source of something unwanted (a parking ticket, compliments of the city).

In hotels, restaurants and theaters the free sense is often shortened to comp: Merriam-Webster records the noun, meaning a complimentary ticket, from 1871, and the verb, to provide something free, from 1951. In publishing, though, comps also means comparable titles, so spell it out in a query letter if you mean free copies.

One more fixed phrase keeps the i: the complimentary close, the words such as Sincerely yours that come just before the signature of a letter. It is a courtesy, not a completion. The guides to ending an email and choosing email sign-offs cover what to put there.

Examples in sentences

Complementary and complement (completing, matching):

1. The two founders have complementary strengths: she sells, he builds.
2. Blue and orange are complementary colors, so the cover pops.
3. Angles of 35 and 55 degrees are complementary.
4. A crisp cider complements the pork belly.
5. The maps complement the text without repeating it.
6. Her dry humor is the perfect complement to his earnestness.
7. The ship sailed with a full complement of crew.
8. The two studies are complementary: one measured cost, the other time.
9. The new hires complement the existing team rather than duplicate it.
10. Each DNA strand is complementary to the other.
11. The museum's shop sells books complementary to the exhibition.

Complimentary and compliment (praise):

12. The reviewer was complimentary about the dialogue.
13. He complimented her on the speech, and she believed him.
14. Please pass my compliments to the chef.
15. That's the nicest compliment anyone has paid my handwriting.
16. The board's comments were less than complimentary.

Complimentary (free):

17. Breakfast is complimentary for guests staying two nights or more.
18. Speakers receive two complimentary passes for the conference.
19. Each contributor gets a complimentary copy of the anthology.
20. The hotel offers a complimentary shuttle to the airport.
21. Coffee refills are complimentary until noon.

Both in one sentence:

22. The complimentary wine was a nice complement to the cheese.
23. She complimented the designer on the complementary colors.
24. The reviews were complimentary, and the two books proved complementary.

Mistakes to look for

• complementary breakfast → complimentary breakfast (free)
• complimentary colors → complementary colors (they complete each other)
• complimentary medicine → complementary medicine (used alongside)
• a full compliment of staff → a full complement of staff
• Dessert, complements of the chef → Dessert, compliments of the chef
• The tie compliments your suit. → The tie complements your suit. (Unless the tie can talk.)
• a complementary close → a complimentary close

The rule holds across the whole family. Anything that completes, matches or fills a number takes the e; anything that praises, greets or comes free takes the i.

This pair slips into copy written fast and read by everyone: the event page, the welcome email, the menu card. Before it goes out, ask the Strokes Assistant to list every complement, compliment, complementary and complimentary in the document with its sentence, and run the two quick tests down the list. Other sound-alike pairs worth the same check: principal and principle and further and farther.

Quick quiz: complimentary or complementary?

Fill each gap with complementary, complimentary or, where the sentence needs a noun, complement or compliment.

1. Valet parking is ____ for diners.
2. Red and green are ____ colors.
3. Her colleagues were very ____ about the report.
4. A 40-degree angle and a 50-degree angle are ____.
5. The hotel left a ____ bottle of water by the bed.
6. Their research interests are ____: one studies soil, the other roots.
7. She ended the letter with a formal ____ close.
8. The hospital's ____ medicine program runs alongside its regular clinics.
9. The crew reached its full ____ in May.
10. Thank you; that's very ____ of you to say.

Where the two words come from

Both words go back to the Latin complere, “to fill, make up, carry to completion.” Complement is the direct descendant, from Latin complementum, something that fills out or completes. Compliment took the long way round: Merriam-Webster traces it through French and Italian to the Spanish cumplimiento, an offer made for the sake of etiquette, and dates it to 1598. For a while English didn't keep the spellings apart. Merriam-Webster still lists an obsolete sense of the verb complement meaning compliment, and one scene of Shakespeare can show both spellings:

In this text (editions differ), the King describes the Spaniard Armado as a man of elaborate courtesies, spelled with an e. Minutes later he reads aloud Armado's letter, which ends with that overheated sign-off spelled with an i: a complimentary close long before anyone called it one.