Many years ago, a law firm in Chicago asked me to be an expert witness on the difference between that and which. The issue concerned a selling manufacturer’s non-compete clause. That at the beginning of the clause meant zero dollars for the buyer; a comma followed by which meant millions of dollars for the buyer. But the clause read "which" with no comma.

The difference between that and which might be the most confounding piece of grammar in the English language, but it doesn’t have to be. Here’s what you need to know: Grammarians call the words following that or which a “relative clause.” That relative clause either "restricts" what it modifies, or it "does not restrict" what it modifies. The writer tells us which it is by the word chosen to introduce the clause.

That at the beginning restricts; it means that the writer wants the relative clause to distinguish one thing from a universe of like things. Which at the beginning does not restrict; it means that the writer addresses only one thing to begin with, so there is nothing to restrict; the relative clause simply adds information.

The cabin that sleeps eight people is farther up the mountain.

Lots of cabins on that mountain, but only one sleeps eight people, and it’s farther up.

The cabin, which sleeps eight people, is farther up the mountain.

Only one cabin on this mountain, farther up, and, by the way, it sleeps eight people.

The confusion arises when the writer uses which with no comma:

The Pioneer Bank which sits at Fifth and Pine has a free ATM.