An Obscure Bit of Grammatical Knowledge: The Difference Between "That" and "Which"

We can all doff our metaphorical hats in gratitude to Kathleen in the wilds of Akita-ken, who prompted the unearthing of this arcane grammatical rule.

The difference between "that" and "which": "that" is used with restrictive clauses, and "which" is used with unrestrictive clauses.

What does that mean?

It means one uses "that" when one needs to specify something about the subject (i.e. "restrict" the subject to a smaller subset) to make clear what one is referring to.

For example: Please fetch me the human head that I collected yesterday morning.

In this sentence, the speaker is specifying the human head he would like to have fetched: not the ones he collected the day before yesterday, not the ones he collected yesterday afternoon. He would like the one he collected yesterday morning.

One uses "which" when one doesn't need to specify anything about the subject.

For example: I once had a lovely human head which I displayed on the mantelpiece next to the portrait of my aunt.

In this sentence, it isn't necessary for the speaker to give more details about the human head for us to know what human head he is talking about. We know already that he is referring to a "lovely" human head which he once owned. The description that follows ("which I displayed on the mantelpiece next to the portrait of my aunt") is interesting, but unnecessary.

Other examples:

1. Take back the human head that you gave me on the occasion of our first year wedding anniversary and get out, you ruffian-knave!!!

2. The human heads which filled the wheelbarrow made a pleasant thumpity-thump sound as Priscilla carted them off to the vegetable-patch.