Question 1

What is the difference between Principal and Principle?

Accepted Answer

Principal means main or chief, and as a noun it's a person in charge (or a sum of money). Principle is a rule, law or belief. They're pronounced the same, so spelling is the only signal: the head of a school is the principal; the reason you won't lie is a principle.

Question 2

How do I know if I mean Principal rather than Principle?

Accepted Answer

Readers who land here meaning Principal rather than Principle are usually searching for phrases like “principal or principle”, “school principle or school principal”, “in principal or in principle”.

Aspect	Principal	Principle
Parts of speech	Adjective and noun	Noun only
Core meaning	Main, chief; the person in charge	A rule, theory or moral belief
Example	'Tourism is the principal industry.'	'It's against my principles.'
Fixed phrases	'Principal conductor', 'loan principal'	'In principle', 'on principle', 'first principles'

Side by side

Which one did you mean?

Principal

Read either one in full