Complement vs. Compliment, Principal vs. Principle

Some misspellings are flagged at once by any spellchecker. A different, sneakier kind is never flagged at all, because both spellings are real words, just the wrong one for the sentence. These pairs need you to know the difference, since no red underline is coming to rescue you. This guide covers two of the most common: complement and compliment, and principal and principle.

Complement and compliment

The two words differ by one vowel, and they share no meaning at all.

Complement is about completing or improving something. Cambridge Dictionary defines the verb as to make something else seem better or more attractive when combined with it, and to help make something or someone more complete or effective. As a noun, it can mean a thing that completes something else.

Examples: "The wine was a good complement to the meal." "Her calm manner complements his energy." "The two designs complement each other."

Compliment is about praise. Cambridge Dictionary defines the noun as a remark that expresses approval, admiration or respect, and the verb is to say something like that to someone.

Examples: "She gave me a compliment on the presentation." "He complimented the chef on the dessert." "That is a high compliment, coming from her."

A memory aid: complement has an E, like complete. A complement completes something. Compliment has an I, like I like it. A compliment is a nice thing to hear.

A replacement test: if you can say completes or goes well with, it is complement. If you can say praise or nice remark, it is compliment. "Her scarf ___ her coat" works with goes well with, so complements. "He paid her a ___ on her talk" works with a nice remark, so compliment.

A third form to watch: complimentary (with an I) means expressing praise, or, in a different sense, given free: "a complimentary glass of water". Complementary (with an E) means that two things go well together or complete each other: "complementary colors". A complimentary ticket is free, and a complementary skill fills a gap.

Principal and principle

The same problem appears with principal and principle. They sound identical, and they have unrelated meanings, apart from sharing a Latin root.

Principal has several uses. As an adjective, it means first in order of importance: "the principal reason for the delay", "the principal character". As a noun, it is the person in charge of a school, college or university, as Cambridge Dictionary lists. And in finance, it is an amount of money that someone has invested or lent, on which interest is paid.

Examples: "Cost was the principal concern." "The principal welcomed the new students." "The loan's principal was repaid in five years."

Principle is a noun only, and it means a basic idea, rule or belief. Cambridge Dictionary gives two main senses: a basic idea or rule that explains how something works, and a moral rule or standard of good behavior.

Examples: "The principle behind a lever is simple." "She refused on principle." "In principle, I agree, though the details matter."

A memory aid: a school principal is your pal, and the adjective principal often describes a main person or thing. A principle is a rule: both end in -le.

A replacement test: if you can say main or head, it is principal. If you can say rule or belief, it is principle. "The ___ cause of the problem" works with main, so principal. "A matter of ___" works with belief, so principle.

Why spellcheck cannot save you

A spellchecker flags strings of letters that do not form a word. It generally does not flag a real word used in the wrong sense, which is exactly the error both pairs create. "He gave me a nice complement" will pass without a mark, and the sentence will still be wrong. So will "the school principle spoke to us".

Grammar checkers do better, since some of them look at the surrounding words, but they miss many cases and sometimes suggest the wrong fix. Treat any tool as a second pair of eyes, and not as a guarantee.

Catching these errors when proofreading

The best defense is a deliberate step. After you finish a draft, search the document for each of these words in turn, and check each use against the replacement test. A search for four words takes less than a minute, and you only need to look at the places where they appear. Reading the draft aloud helps less than usual here, because the words sound the same, so reading the text on the page with your finger or a ruler, one line at a time, helps more.

It also helps to keep a short list of your personal trouble pairs. Most writers repeat the same few mistakes. If you notice you often mix up one pair, put it at the top of your checklist.

Related pairs worth knowing

Once you start looking, English has a long list of pairs where both spellings are real. Stationary (not moving) and stationery (paper and pens), discreet (careful about what you say) and discrete (separate), advice (a noun) and advise (a verb), lose and loose, accept and except, breath and breathe, practice (noun) and practise (British verb), and whose and who's. Common English homophones covers pairs that sound the same, and affect vs. effect and imply vs. infer look at pairs where the meaning is what matters.

A practice set

Choose the correct word. Answers follow.

1. Red wine is a good (complement / compliment) to roast beef.
2. She was pleased by the (complement / compliment) about her work.
3. The (principal / principle) reason for the change was cost.
4. He refused to lie, as a matter of (principal / principle).
5. The school (principal / principle) spoke at the assembly.
6. The hotel gave us a (complimentary / complementary) upgrade.
7. The two plans are (complimentary / complementary), and we should use both.
8. In (principal / principle), the idea is sound.

Answers: 1. complement. 2. compliment. 3. principal. 4. principle. 5. principal. 6. complimentary. 7. complementary. 8. principle.

Making the right choice automatic

Understanding the rule is the first step, and using it correctly a handful of times is what makes it stick. Choose one pair, write three sentences of your own for each word, and test yourself on them the next day and again a week later. Spaced testing beats rereading, as how spaced repetition works and active recall versus rereading explain.

How Wordabulary fits in

Wordabulary shows each word with a definition and an example sentence, so you can see complement and compliment in use side by side. You can look both pairs up in the word library, and the quiz formats include a typing mode that asks you to produce a word from its meaning. It works best alongside your own writing and proofreading.