March sentences use adjective clauses, which start with who, which, or that and modify a noun or pronoun directly in front of them. People who use them correctly use who only with people, which with places and things but never with people, and that with places or things and rarely with people. Adjective clauses that are necessary to the sentence don't take any commas, but other adjective clauses, ones that might not be necessary to the sentence, take a comma before and after the clause.
Diet Coke and Mentos
Greek alphabet song