Principal vs principle: meanings, examples and memory tricks

Principal means first or most important, and as a noun it names the leading person or thing: the school principal, the principal in a business deal, the principal on a loan. Principle is only ever a noun, and in almost every use it means a rule, a belief or a law of how something works: the principle of free speech, a person of principle, the principle behind a lever. The quick test: if you could swap in main or chief, write principal. If you could swap in rule or belief, write principle.

Both fit naturally in one sentence: The principal explained the principle behind the new phone policy. They are pronounced the same, /ˈprɪnsəpəl/, and both are real words, so a spell-checker lets either one through. That is why "principle amount" and "the principle of the school" turn up in reports, contracts and grant applications that have otherwise been read carefully.

Principal vs principle at a glance

What principal means

Principal came into English through Anglo-French from the Latin principalis, "first, chief", and every one of its meanings keeps that idea of the first or leading thing. As an adjective it means the main one of several:

• The principal cities of the region are the largest or most important ones.
• The principal clarinet plays first chair in an orchestra, and a principal dancer holds the top rank in a ballet company.
• The principal parts of a verb are the forms you build its other forms from: go, went, gone.
• Principal component analysis, a statistical method, finds the main components that explain most of the variation in a data set.
• Principal photography is the main phase of filming a movie, after preproduction and before post.

As a noun, principal names the leading person or thing:

• The head of a school. American schools have a principal and often an assistant principal. British schools more often say head teacher.
• The original sum of money. In a loan or an investment, the principal is the amount borrowed or put in, as opposed to the interest it earns or costs. A mortgage statement shows "principal and interest" separately.
• The person an agent acts for. In law and business, an agent acts on behalf of a principal: a client, an employer, the party who gives the instructions.
• A senior person in a firm. Many consulting, architecture and investment firms use principal as a job title for a senior partner or owner.
• A lead performer. "The principals" in a cast are the leads, as opposed to the chorus or the extras.
• The person who commits a crime. Criminal law distinguishes the principal from an accessory, who helps before or after the fact.

The adverb is principally, "mainly": The grant is principally for equipment. There is no adverb from principle.

What principle means

Principle came into English from the French principe, from the Latin principium, "beginning". A principle is the thing everything else is built on, and in everyday use the word has four related senses:

• A basic truth or theory that other reasoning rests on: the principles of economics, the first principles of a subject.
• A rule of conduct or a moral belief. A person of principle has rules they won't break. To refuse on principle is to refuse because of such a rule, not because of the details of this case.
• A law of nature or the way a device works: Archimedes' principle, Heisenberg's uncertainty principle, the principle of the lever, the principle behind a refrigerator.
• A named rule of thumb: the Pareto principle (the 80/20 rule), the Peter principle (people rise to their level of incompetence), the precautionary principle, and in computer security the principle of least privilege: give every program and user only the access the job needs.

One more sense turns up in chemistry and pharmacology, and it is the exception that trips the swap test. The active principle of a drug or a plant is the ingredient that produces its effect. "Main ingredient" seems to fit, but the spelling is principle.

The adjective is principled (a principled decision), and its opposite is unprincipled. Principle itself never works as an adjective, which is a useful check: in front of another noun, as in "the princip___ reason", the word you want is almost always principal.

Set phrases that trip people up

Most mistakes happen inside fixed phrases, where the meaning isn't obvious at a glance. These are the ones worth memorizing.

Principal and principle in sentences

Twenty-five examples, grouped by sense. Read each one with the swap test.

Principal, adjective (main):

1. The principal cause of the outage was a misconfigured backup job.
2. Lagos is Nigeria's principal commercial city, though Abuja is the capital.
3. Our principal concern is that readers won't reach chapter two.
4. She joined the orchestra as principal cello in 2019.
5. Principal photography wrapped in March, and the edit took a year.
6. The study's principal finding held in every age group.

Principal, noun (person or sum):

7. The principal asked parents to pick children up by 3:15.
8. Mr. Okafor was assistant principal for six years before he took over.
9. An extra payment each month goes straight to the principal.
10. The loan's principal was $40,000; the interest added another $9,000.
11. As the principal investigator, Dr. Reyes signs the grant report.
12. The broker acted for an undisclosed principal.
13. She is a principal at a small architecture firm in Denver.
14. The director cast the principals first and the chorus last.

Principle (rule, belief, law):

15. The principle of free speech protects opinions you dislike.
16. He refused the sponsorship on principle.
17. The plan is fine in principle; the budget is the problem.
18. A hot-air balloon rises on a simple principle: warm air is lighter than cool air.
19. Our style guide rests on one principle: write for the reader in a hurry.
20. She is a woman of principle, and her opponents know it.
21. Apply the principle of least privilege to every service account.
22. It's not the money; it's the principle of the thing.

Both in one sentence:

23. The principal explained the principles behind the new grading policy.
24. Our principal objection is a matter of principle, not cost.
25. The bank's guiding principle is to protect the depositors' principal.

The mistakes people actually make

Slips go both ways: principle where principal belongs, often in money and school phrases, and principal where a fixed phrase such as in principle needs principle. The fixes:

• the principle of the school → the principal of the school
• the principle amount of the loan → the principal amount
• principle and interest → principal and interest
• the principle investigator → the principal investigator
• my principal reason is the principal of the thing → my principal reason is the principle of the thing
• agreed in principal → agreed in principle
• a principaled stand → a principled stand

Watch plurals too. "The principals of physics" would mean the heads of the physics department. "The principles of physics" are its laws.

Quick quiz: principal or principle?

Fill each gap, then check the answers below.

1. The ____ called a meeting with the teachers.
2. We support the proposal in ____.
3. The ____ ingredient in the sauce is roasted garlic.
4. Your monthly payment reduces the ____ and covers the interest.
5. Newton's laws are among the basic ____s of mechanics.
6. She wouldn't sign the letter as a matter of ____.
7. The ____ dancers rehearse separately from the corps.
8. Engineers designed the vault on the ____ that no one person should hold every key.

Catch it in your own drafts

Search the document for princip: it finds both words, their plurals and principally in one pass. Run the swap test on each hit, especially in loan agreements, grant proposals and letters to a school.

In a long report, you can also ask the Strokes Assistant to list every sentence that uses either word, so you can check them all in one place.

The same check works for led and lead, which also sound alike, and for advice and advise, which differ by one letter. A pair with two correct spellings, like judgement and judgment, is a style decision instead; note it on your style sheet once and apply it everywhere.