principal / principle
One is a person or the main thing. One is a rule.
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Wrong: It’s a matter of principal.
Right: It’s a matter of principle.
A rule or belief. -ple, like rule.
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Wrong: Our principle concern is cost.
Right: Our principal concern is cost.
Main, chief. -pal.
The mnemonic
A principle is a rule. Both end in -le, and that’s the only one you need to remember—everything else is principal.
The old school mnemonic works too: the principal is your pal. Same spelling covers the main thing (principal reason) and the money in a loan (principal and interest).
The one that gets noticed
A matter of principal is the common failure, and it reads as a spellcheck-clean typo in exactly the kind of sentence where you were trying to sound serious.
Now go stand on one.