Core rule
"Principal" is usually a noun meaning "the most important person or thing" or an adjective meaning "main." "Principle" is always a noun meaning "a fundamental rule or belief."
- The principal reason we lost the client was late delivery.
- She refused to cut corners; it was a matter of principle.
- The school principal called about the field trip.
The memory trick that actually works
"Principal" ends in -pal, like "pal": a school principal used to be introduced to students as your "pal" (even if that was never quite true). If the word is a person or the main/most important version of something, it takes the -pal spelling. "Principle" ends in -ple, like "rule": if you can swap in the word "rule" and the sentence still makes sense, it is "principle."
The one that trips people up: finance
In lending and investing, "principal" is the noun for the original amount of money borrowed or invested, separate from interest or returns. This is the single most common professional-writing error with this pair, because "principle" feels more natural next to a serious word like "investment." It is not: you pay down the principal, not the principle.
- Correct: Extra payments go entirely toward the principal, not the interest.
- Incorrect: Extra payments go entirely toward the principle.
Editing checklist
Read the sentence back and try substituting "main" or "chief." If it still works, you likely need "principal." Try substituting "rule" or "belief" instead; if that works, you need "principle." If you are writing about a loan, a mortgage, or an investment amount, it is almost certainly "principal."