At the end of an argument, after you've said what's on your mind, have you said your piece or said your peace?

This is a tricky one! But if you mean that you've said what you're thinking and that's that, you've said your piece. The word piece in the sense of "an opinion" has been around since the late 1500s. It's used this way when you give someone a piece of your mind or if you say your piece. When you've said your piece, you've clearly stated your opinion on a topic.

These examples use the phrase correctly:

You've said your piece, you've given your reasons, and now your friend will make her own decision. (Slate)

Let her say her piece, then choose whether you then have any interest in saying yours. (Washington Post)

So why do we see the expression sometimes misspelled as say your peace? For one thing, piece and peace sound exactly the same. People might also confuse this expression with hold your peace. To hold your peace or keep your peace means to stay silent — which is the opposite of saying your piece. Hold your peace is often heard in Christian wedding ceremonies, when the officiant says that anyone who objects to the marriage must "speak now or forever hold your peace."

Though there are reasons to confuse piece with peace in this expression, say your piece is how the phrase is traditionally spelled. Here are a couple of examples of publications that erroneously gave peace a chance:

"He said his peace, and I just heard him out. Obviously, he brings a lot to our team, and we'd love to have him back this offseason and start working with him." (US News)

He wants a chance to say his peace about the event — and the long-lost love — that sent him into seclusion in the first place. (Time)

If you speak your mind, you say your piece. If you're going to keep quiet, you hold your peace. And now we've said our piece on the matter.