Principal vs principle: what each word means, a trick to remember it, and examples
Principal means main or most important, and it is also a person in charge or a sum of money. Principle is a rule or a belief. If you can swap in "main," you want principal. If you can swap in "rule" or "belief," you want principle. Here are the meanings, a memory trick, examples and the phrases that trip people up.
What each word means
- Principal (adjective): main, chief, most important. "The principal reason I wrote this book."
- Principal (noun): a person in charge. The head of a school, or a partner or owner of a business. "She met with the principal."
- Principal (noun): money. The original amount lent or invested, on which interest is calculated. "Pay down the principal first."
- Principle (noun only): a rule, basic truth or guiding belief. "The book is built on seven principles." "It's a matter of principle."
The two words share a root. Dictionary.com traces principal to the Latin prīncipālis, "first, chief," and principle to prīncipium, "beginning, origin, basis." That is why they look alike, and why one became the main thing and the other became the basic rule.
A trick to remember it
Dictionary.com offers the classic one: the principal of your school is your pal. A second one that works for writers: principle and rule both end in -le.
And a quick test while editing: if the word is describing another noun (the ___ reason, the ___ character, the ___ investor), it must be principal. Principle is never an adjective.
Examples
| Sentence | Word | Why |
|---|---|---|
| The ___ argument of chapter 3 is that habits beat goals. | principal | Means "main" |
| The book lays out five ___ for leading a small team. | principles | Means "rules" |
| My high school ___ was the first person to read my essays. | principal | Head of a school |
| She refused on ___. | principle | A belief about right and wrong |
| Extra payments go toward the ___ on the loan. | principal | The original sum of money |
| We agreed in ___ to a second edition. | principle | Fixed phrase: "in theory" |
| The firm's two ___ founded it in 2009. | principals | Owners or partners of a business |
Examples written by Scribe Media.
Phrases that trip people up
- In principle (in theory), never "in principal."
- On principle (because of a belief), never "on principal."
- Principal investigator, principal dancer, principal photography: all mean the chief or main one, so principal.
- Guiding principles, first principles, core principles: all rules or beliefs, so principle.
Checking your manuscript
Spellcheck will not catch this one, because both are real words. Business and self-help books are especially prone to it, since they often talk about principles and the principal idea in the same paragraph. Search your file for "princip" and check each one with the swap test. A copy editor will catch the rest; see what a copy editor does. For other pairs that spellcheck misses, see lay vs lie and who vs whom.
Start here: search your manuscript for "princip." For each hit, try swapping in "main." If it fits, the word is principal. If "rule" or "belief" fits instead, it is principle.
Common questions
What is the difference between principal and principle?
Principal means main or most important, and as a noun it means a person in charge, such as the head of a school, or the original sum of money in a loan or investment. Principle is always a noun and means a rule, a basic truth or a guiding belief.
Is it school principal or principle?
Principal. The head of a school is the principal. A popular memory trick: the principal is your pal.
Is it in principle or in principal?
In principle. The phrase means in theory or as a general idea ("We agree in principle"), so it uses principle, the word for a rule or basic idea.
Is it the principal reason or the principle reason?
The principal reason. When you mean main or most important, the word is principal. Principle is never an adjective.
Is principal on a loan spelled principal or principle?
Principal. The original amount of money borrowed or invested, on which interest is calculated, is the principal.
Sources: Dictionary.com, Principal vs. Principle (meanings, Latin roots, the "pal" memory trick) and Wiktionary entries for principal (main; head of a school; money lent or invested; partner or owner of a business) and principle (fundamental assumption or guiding belief; moral rule), all read November 2, 2026. The "-le" trick, examples, test and guidance were written by Scribe Media. Published by Scribe Media.