Confusion about the meanings of principle and principal is quite common. Let’s clarify the issue once and for all so you can use these terms correctly in your writing.

Principle is a noun. According to Merriam-Webster, it means a “law, doctrine, or assumption” (“Principle”). In other words, principle refers to some kind of rule.

According to the Hippocratic oath, physicians promise to uphold certain principles of professionalism and ethics.

Let’s review the principles underlying space flight.

Principal can be a noun or an adjective but means similar things in either grammatical role. As a noun it refers to “the main thing” or “head person,” and as an adjective it means “main” (Cook 191) or “chief” (“Principal”). Below is an example of principal as a noun and as an adjective.

She was recently appointed principal of the school.

What are the principal reasons you decided to study abroad?

Works Cited

Cook, Claire Kehrwald. Line by Line: How to Edit Your Own Writing. Houghton Mifflin Harcourt, 1985.

“Principal, Adj. (1).” Merriam-Webster Unabridged, 2023,

“Principle, N. (1).” Merriam-Webster Unabridged, 2023,

Photo of Michael Simon

Michael Simon

Michael Simon is associate editor at the MLA. He received an MFA in poetry from Columbia University and a BA in comparative literature from Brown University. Before coming to the MLA, he worked as an editor for several academic publishers.