Which vs. That - Grammar Rules | Brian A. Klems, WritersDigest.com

If you've been confused about choosing between which and that, this blog provides a clear explanation. (And yes, making that choice is easy to do. And it is important to do, because using the correct word helps readers understand the information in your sentence.)

I was surprised that Kiems did not emphasize one important difference between the words: how to punctuate them. On the other hand, he punctuates his examples sentences correctly.

My style guide, Garbl's Editorial Style Manual, also provides advice on using that and which. But here's what it says about punctuating the words:

Which clauses are always set off with commas (or sometimes dashes or parentheses), and that clauses aren't. Essential that clauses cannot be cut without changing the meaning of a sentence. Don't set off an essential clause from the rest of a sentence with commas. Nonessential which clauses can be dropped without altering the meaning. Set off a nonessential clause with commas.

Examples: The camera that is broken is in the shop (tells which one). The camera, which is broken, is in the shop (adds a fact about the only camera in question).