Quick answer: Principal can be a noun (the head of a school) or adjective (main, most important): The principal reason for the delay. Principle is always a noun meaning a fundamental rule, moral belief, or law of science: the principle of equality. Memory tip: the school princip-AL is your p-AL. For a fuller definition, see the Cambridge Dictionary entry for ‘principal’.
Comparison Table
| Word | Part of Speech | Key Meaning | Example |
|---|---|---|---|
| principal | noun / adjective | main, most important; head of a school or organisation | The principal reason; the school principal. |
| principle | noun only | fundamental rule, moral belief, or law of science | The principle of equality. |
Using Principal
Principal has two main uses. As an adjective, it means main, chief, or most important. As a noun, it refers to the head of a school, college, or organisation — or, in finance, the original sum of money invested or borrowed.
Principal as an Adjective (= main)
The principal reason for the delay was a lack of funding.
Her principal concern was the safety of her students.
Agriculture is the country’s principal industry.
Principal as a Noun (= head person or sum of money)
The school principal gave a speech to the parents.
She is the principal of a large secondary school.
You must repay the principal plus any interest accrued.
Using Principle
Principle is always a noun. It refers to a fundamental truth, rule, or belief that guides behaviour or explains how something works. It is commonly used in moral, scientific, and legal contexts.
She stood on principle and refused to sign the contract.
Newton’s first principle of motion states that an object at rest stays at rest.
The principle of equal pay is widely accepted in modern workplaces.
He agreed in principle but wanted to discuss the details further.
Common Phrases with Principle
Several fixed expressions always use principle (never principal):
- in principle — in theory, as a general idea: We agree in principle.
- on principle — because of a moral rule: She refused on principle.
- a man/woman of principles — someone with strong moral values.
Memory Trick
Here is the easiest way to remember the difference:
- Princip-AL = a person (your p-AL). The school principal is your pal.
- Princip-LE = a ru-LE. Principle ends in -le, just like rule.
Common Mistakes
✗ The principle reason for the decision was cost.
✓ The principal reason for the decision was cost.
Principle is always a noun. Use principal when you need an adjective meaning “main”.
✗ We agree in principal.
✓ We agree in principle.
The fixed phrase is always “in principle” (= in theory). Principal is a person or adjective, not used in this phrase.
✗ She is a woman of strong principals.
✓ She is a woman of strong principles.
Moral beliefs and values are always principles (noun). Principals (plural noun) refers to heads of schools or key parties in a deal.
Tips to Avoid Confusion
- Ask yourself: is this a person or adjective? Use principal.
- Ask yourself: is this a rule, belief, or law? Use principle.
- If you can replace the word with main or chief, write principal.
- If you can replace the word with rule or belief, write principle.
- Remember: in principle and on principle always use principle.
Also confusing: Accept vs Receive, Advise vs Advice, Affect vs Effect. Browse all confusing word pairs →
Quick Check: Principal or Principle?
Four fill-in-the-blank questions — choose the correct word for each sentence.
1. “The ___ concern is student safety.”
2. “He refused on ___ — it was a moral issue.”
3. “The school ___ addressed the parents at the assembly.”
4. “The ___ of equal pay is widely accepted.”
Frequently Asked Questions
What is the difference between principal and principle?
Principal can be a noun (the head of a school or organisation) or an adjective meaning main or most important. Principle is always a noun meaning a fundamental rule, moral belief, or scientific law. The two words sound identical but have completely different meanings.
Is principal a noun or adjective?
Principal can be both. As a noun it means the head of a school or the most important person in a group: “The principal called a meeting.” As an adjective it means main or most important: “The principal reason was cost.”
Can principle ever be used as an adjective?
No. Principle is always a noun. You cannot say “the principle reason”. If you want an adjective, use principal: “the principal reason”. A common mistake is to write “in principle” correctly but then confuse the spelling in adjectival use.
What does “in principle” mean?
“In principle” means in theory or as a general idea, even if the details are not yet agreed: “We agree in principle but need to discuss the terms.” This phrase always uses principle (the noun meaning rule or belief), never principal.
What does “on principle” mean?
“On principle” means because of a moral rule or personal belief, regardless of practical consequences: “She refused on principle.” Again, this always uses principle (noun), not principal.
How can I remember the difference between principal and principle?
Use this memory trick: the school princip-AL is your p-AL (pal). Both principal (the person) and pal end in -al. Principle (the rule) ends in -le, just like rule. So: princip-AL = a person; princip-LE = a ruLE.
What is a principal in finance?
In finance, principal refers to the original sum of money borrowed or invested, not including interest: “You must repay the principal plus interest.” This is the noun use of principal meaning the main or primary amount.
Can principal mean first in rank?
Yes. As an adjective, principal means first in rank, importance, or degree: “She played the principal role in the film.” “The principal export of the country is oil.” It is synonymous with main, chief, or primary in these uses.
What are examples of principle used correctly?
Correct uses of principle: “The principle of equal pay is widely accepted.” “Newton’s first principle of motion states that an object at rest stays at rest.” “He is a man of strong principles.” “She agreed in principle but wanted more details.”
How do principal and principle appear in English exams?
In Cambridge and IELTS exams, confusing principal and principle is a common spelling and vocabulary error that lowers your score. Remember: principal (adj/noun) = main/head person; principle (noun only) = rule or belief. Check which part of speech you need before choosing.