Question: “I get e-mails all the time in which people mix up the words ‘principle’ and ‘principal.’ I even see them misused on the news! Maybe you can write a column to clear up some of the confusion.”
Answer: What an apropos question – I just spent the entire day with the word “principal” on the tip of my tongue after a phone call from my car dealership (They offered me the opportunity to keep my same monthly payments, but on a larger loan. What a deal!)
We’ll look at “principle” first, since it is the simpler of the two.
“Principle” derives from the Latin principium, meaning “source,” and it has maintained that definition in English. We use “principle” to mean “a fundamental source or basis of something” or “a fundamental truth or proposition.” For example, free speech is a principle of American democracy.
“Principle” is also always a noun – thus being (slightly) simpler than “principal.”
When I was in elementary school, we had a little mnemonic to help us remember the spelling of “principal:” “The principal is always your ‘pal.'” This didn’t turn out to be true, necessarily, for all of my classmates, but its usefulness as a grammar tool holds up.
“Principal” derives from the Latin principalis, or “first; original.” From there we end up with our modern definitions in the adjectival form: “first in order of importance” or, in economics, “denoting an original sum invested or lent.” As an extension of the former definition, we get “principal” as a noun meaning “the most important or senior person in an organization or group;” “the head of a school, college or other education institution;” or “the leading player in each section of an orchestra” – the “first” person, more or less. From the latter definition we get “principal” as a noun meaning, as you may have guessed, “an original sum invested or lent.”
The thing to take away from this is that if you are talking about the source of something, or a fundamental truth, you want “principle.” Otherwise, you want “principal” – and whether you use it as an adjective or noun should come naturally based on the context.