[rendall]

English has "thou", which was the singular second person pronoun, and "you" which was the plural second person pronoun, but also used as a more formal, polite singular second person pronoun. Through a kind of formality inflation, everyone started using "you" all the time and abandoned "thou". Contemporary standard English doesn't have a plural second person pronoun, but needs one, and so "y'all" (or "ya'll") is being used now in some regions.