Principal vs Principle
Principal can be an adjective meaning main or most important, or a noun meaning the head of a school, the leading performer, or the original sum of a loan. Principle is always a noun meaning a fundamental rule, truth, or belief. They sound identical and look almost identical — and the cleanest memory trick is that the head of your school is your pal.
Last reviewed on 2026-04-27.
Quick Comparison
| Aspect | Principal | Principle |
|---|---|---|
| Part of speech | Adjective or noun | Noun only |
| Adjective meaning | Main, most important | (not used as adjective) |
| Common noun meanings | Head of school; leading performer; original loan amount | Fundamental rule, truth, or belief |
| Memory aid | "-pal" — the principal is your pal | "-ple" — like a rule |
| Examples | The principal reason; the school principal; loan principal | A guiding principle; principles of physics |
| Plural | Principals | Principles |
Key Differences
1. Different parts of speech
Principal can be an adjective meaning main or most important: "the principal reason," "the principal cause." It can also be a noun with several meanings: head of a school, lead performer, or the original amount of a loan (separate from interest).
Principle is always a noun. It means a fundamental rule, truth, doctrine, or moral belief: "the principles of physics," "a man of principle," "the principle of equality before the law."
2. The classic memory trick
Princip-al ends in "-al" — and a friendly version of the school principal is your pal. "The principal is my pal." That sticks for many people forever.
Princip-le ends in "-le" — like "rule" (almost; both end in a vowel + le). Principles are rules.
3. Worked sentences
"The principal reason for the delay was the weather." "Mrs. Chen is the principal at our local school." "Each payment includes interest plus a portion of principal." "The dancer played the principal role."
"She refused on a matter of principle." "This violates the principle of fair use." "The principles of thermodynamics are well established." "He stuck to his principles."
4. A nuance about "principal"
As an adjective, principal almost always means "main" or "primary," not "first." "The principal violinist" is the lead violinist; "the principal city" is the most important city.
As a noun, the meaning depends on context: school principal (educator), loan principal (financial), a principal in a business deal (a primary party).
5. Common slips
"It was a matter of principal." Wrong — should be principle (a rule or belief).
"The principle of the school addressed the assembly." Wrong — should be principal (the person in charge).
6. Both can appear together
A correct sentence with both: "The school principal stood by her principles." The first is the head of the school; the second is the rules she lives by.
Or a financial example: "Her principal investment was guided by sound principles." Different words; both correct.
When to Choose Each
Choose Principal if:
- Adjective: main, most important, primary.
- Noun: head of school, lead performer, original loan amount, primary party in a deal.
- Anywhere the meaning is "the chief one."
Choose Principle if:
- Fundamental rules: scientific principles, legal principles, ethical principles.
- Beliefs: "a person of strong principles," "on principle."
- Doctrines: "the principles of design," "the principle of separation of powers."
Worked example
"The school principal stood by her principles when she refused to bend the attendance rules — even when the parents complained, even when the school board pressured her. The principal reason she gave was simple: rules apply to everyone equally, including the principal's own family." Three uses of the two words; two of them adjectives or nouns related to the school, one of them a noun about beliefs.
Common Mistakes
- "They're interchangeable." They're not. Different parts of speech (mostly) and different meanings.
- "Principal always means the school principal." It's also an adjective and applies in finance and business.
- "Principle can be an adjective." No — it's only a noun.