Comma Magic

Reporters who do not read the style guide should not complain about their
editors.

Reporters, who do not read the style guide, should not complain about their
editors.

Big difference, huh? (I had forgotten about this example until I rediscovered it in a style guide yesterday.) The rule at play: Use commas to set off nonrestrictive clauses -- i.e. clauses that don't restrict or narrow down the subject. Use no commas when the clause is designed to do just that -- i.e. narrow down the set of reporters to just those who don't read the guide (instead of suggesting that none of them do.)