Principal vs principle: main thing or rule?

The rule in one table

Writers' Digest explains the key grammar point: while principal can be used as an adjective and noun, principle can only be used as a noun (though principled can be used as an adjective). It adds that principal can refer to a person, while principle can only refer to something a person has.

Trinka points out that the words principal and principle sound identical but have completely different meanings and spellings, so the sound will not help you. You have to decide from the meaning.

Principal: main, head, or original amount

Principal has three uses that matter for most learners.

1. As an adjective: main or most important. Cambridge's example: the principal reason for the failure to take action was poor communication between government departments. Here principal means the most important reason.

• the principal reason, the principal cause, the principal source
• the principal ingredient in a recipe
• the principal dancer in a ballet company

2. As a noun: the head of a school or college. Cambridge notes that we can use principal as a noun to mean the head of a school or college (especially in American English), and gives the example: the college principal made a speech congratulating all the students who were graduating in that year.

• the school principal, the principal of the college
• In British English, you will more often hear headteacher or head for a school, and principal for a college or university.

3. In money: the original sum. In finance, the principal is the amount you borrow or invest, before interest is added. If you borrow a sum of money, you pay back the principal plus interest. This sense is why Trinka stresses that, in a financial context, grasping the difference between principal and principle is crucial.

Because principal describes a person or a main thing, it can be an adjective (principal reason) or a noun (the principal).

Principle: a rule, belief or basic idea

Principle is always a noun. It means a rule, a law or a basic idea.

Cambridge gives two meanings: a rule or theory which explains how something is or works, or a moral rule or guideline.

1. A basic rule or theory: the scientific principles behind even the most complicated computer are relatively simple.
2. A moral rule or belief: he seems to have no principles at all, and is only interested in money. (He has no moral rules or guidelines.)

Common phrases with principle:

• in principle: in theory, as an idea. In principle, the plan is fine, but it is expensive.
• on principle: because of a belief. I never lie, on principle.
• a matter of principle: something important to your beliefs.
• a person of principle: someone with strong moral rules.
• the principle of: the basic idea of something. The principle of fair play.

The related adjective is principled, meaning guided by moral rules: a principled leader. The opposite is unprincipled.

Writers' Digest gives clear correct and incorrect pairs: Correct: He really lacks the principles to be trusted. Incorrect: He really lacks the principals to be trusted. A person has principles, so the word cannot be principals (heads of schools).

The memory trick: -pal vs -ple

Use whichever of these helps you most.

• Principle ends in -le, like rule. Both end in -le, and a principle is a kind of rule. This is the most reliable trick.
• Principal ends in -al, like pal. A school principal is the main person. Think of the principal as your pal in charge.
• Principal is the main person or thing. If you can replace the word with main or head, write principal.

A quick replacement test:

• If main or chief fits, use principal: the main reason → the principal reason.
• If rule or belief fits, use principle: a basic rule → a basic principle.

Check these:

• "She is the school ___." (a person, head) → principal.
• "I have a ___ about lying." (a belief) → principle.
• "This is the ___ reason I called." (main) → principal.
• "It works on the same ___ as a pump." (basic idea) → principle.

A common error appears in this Trinka example, which is wrong: "After being the chemistry teacher at the institute for 10 years, Sammy was made the new principle." Sammy is a person and the head, so the correct word is principal. If a person has a role, the word is principal; if a person has a belief, it is principle.

Principal or principle in finance?

In finance and banking, you want principal.

• Loan principal: the amount you borrowed.
• Interest: the cost of borrowing, added on top.
• Principal and interest: the two parts of a typical repayment.
• Principal investor: the person who put in the main amount.

Examples:

• "The monthly payment covers both the principal and the interest."
• "After five years, you have paid back only a small part of the principal."

The word principle never has this meaning. A financial principle would be a rule about money: the principle of compound interest means the basic idea that interest earns more interest. Here you use principle, because you mean an idea, not an amount.

So the question "Is it principal or principle in finance?" has two answers, depending on whether you mean the amount (principal) or the rule (principle).

If you are preparing for a job interview in finance, practise saying these terms aloud, because they are common in questions. Our guide to HR interview questions and answers shows how to rehearse answers in a calm, clear way.

How to say principal and principle

The two words are homophones: they are pronounced exactly the same, with the stress on the first syllable.

• principal / principle: PRIN-suh-pul

The middle vowel is weak, a short "uh" sound (the schwa), and the final syllable is quick: -pul. Because nothing in the pronunciation tells you which word is meant, listeners decide from the context.

That has a practical effect. In speech, the listener uses context to decide which word you mean, so nobody notices the difference. In writing, the spelling shows your choice, so a wrong spelling is visible at once.

Practice saying them in short sentences:

• "The principal reason is cost."
• "It is a matter of principle."
• "She is the principal of a language school."
• "I follow a simple principle: be honest."

Our guide to the schwa sound explains why the middle syllable in words like principal is so short, and why learners often pronounce it too clearly.

Common mistakes and a quick quiz

Mistakes to avoid

• Wrong: She is the school principle. Right: She is the school principal.
• Wrong: It is a matter of principal. Right: It is a matter of principle.
• Wrong: The principle reason for the delay was the weather. Right: The principal reason for the delay was the weather.
• Wrong: He is a man of principals. Right: He is a man of principles.

Quiz: principal or principle?

1. My ___ concern is safety.
2. The ___ called a meeting for all teachers.
3. I refuse, on ___.
4. The loan ___ is the amount you borrowed.
5. This machine works on a simple ___.
6. She has strong moral ___.
7. The ___ ingredient is flour.
8. He was sent to the ___'s office.

Answers: 1. principal. 2. principal. 3. principle. 4. principal. 5. principle. 6. principles. 7. principal. 8. principal.

Say each full sentence aloud, then write it. For more daily speaking practice that turns small vocabulary rules into habits, see our guide to English speaking practice.