One of the good things about my newish job at the London School of Economics is having to find stuff out. Interesting how often this will be answers to questions that have been hanging around in the back of my mind for some time.

Such as what’s the difference between THAT and WHICH?

So here’s my answer to the THAT/WHICH rule:

That is defining, and Which is not, so you would use That in a restrictive clause.

Er, yes. And…

I know, that helps me like a sack of potatoes too. How about if you use them in this way:

The essays that have to be in on Tuesday should be written in black ink

and

The essays, which have to be in on Tuesday, should be written in black ink.

See?

The THAT example implies that ONLY the essays that have to be in on Tuesday should be written in black ink, and the WHICH example suggests ALL the essays need to be both in on Tuesday and written in black ink.

I’ve looked back through my old stuff and I’ve – mostly – used these two words correctly so the rule must have been implanted many years ago, but I’ve never actually unpacked it before.

Call me anal, but I LOVE all this.

Any grammar questions/rules/tips you want to throw at me before the students put me on the spot will be gratefully received.

Related posts:

How are your sentences?

Writing Do’s and Don’ts

I’m a Winnie the Pooh Fellow