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Principal vs principle

Principle = a rule or belief. Principal = main, a head person, or a loan amount.

principle

noun only: a rule, law or moral belief

Test: Can you swap in “rule” or “belief”?

It’s a matter of principle.
The principles of physics.

principal

adjective: main; noun: head of a school, a lead person, or a capital sum

Test: Can you swap in “main”, “head” or “original sum”?

The principal reason is cost.
The principal called my parents.

A principLE is a ruLE. The principAL is your pAL (or the mAin one).

Test yourself

Five sentences. Apply the test above to each one before you pick.

She refused on _____.
Rain was the _____ cause of the delay.
The _____ gave a speech at graduation.
You pay interest on the _____.
The basic _____ of supply and demand.

Pick an answer for each sentence.

Principal in finance

On a loan or investment, the principal is the original amount, before interest. A mortgage payment is usually split between principal and interest.

Catch it automatically

The grammar checker flags common mix-ups like this one by looking at the words around them, and the sentence checker can explain any sentence you’re unsure about.

Questions people ask

Is it “in principal” or “in principle”?

“In principle”, meaning “in theory” or “as a general rule”.

Can “principle” be an adjective?

No. Only “principal” works as an adjective (“principal dancer”). “Principled” is a related adjective meaning “acting on strong beliefs”.

Other grammar guides

Affect vs effectAffect is usually the verb. Effect is usually the noun.Its vs it’sIt’s = it is / it has. Its = belonging to it.Your vs you’reYou’re = you are. Your = belonging to you.Their, there and they’reTheir = belonging to them. There = place / “there is”. They’re = they are.Advice vs adviseAdvice is the noun (rhymes with “nice”). Advise is the verb (rhymes with “prize”).Then vs thanThan compares things. Then tells you when.Whose vs who’sWho’s = who is / who has. Whose = belonging to whom.Lose vs looseLose = misplace or not win (rhymes with “shoes”). Loose = not tight (rhymes with “goose”).Who vs whomWho does the action (he). Whom receives it (him).I.e. vs e.g.E.g. = for example (some of many). I.e. = that is (the exact thing).Farther vs furtherFarther = physical distance. Further = degree, time or “more”.Everyday vs every dayEveryday = ordinary (an adjective). Every day = each day.Complement vs complimentCompliment = praise. Complement = something that completes or suits.Into vs in toInto = movement inside or a change. In to = “in” (part of a verb) + “to”.Lay vs lieYou lay something down. You lie down yourself.Fewer vs lessFewer for countable things. Less for uncountable amounts.Accept vs exceptAccept = receive or agree to. Except = apart from.To, too and twoTo = direction or verb marker. Too = also / excessively. Two = 2.