“That” and “Which” in Common Usage

It is common today for which to be used with both non-restrictive and restrictive clauses, especially in informal contexts:

• Who ate the cake that I bought this morning?
• Who ate the cake which I bought this morning?

The clause “that I bought this morning” is essential to the meaning – I’m not asking about a cake which I bought yesterday, or this afternoon. Therefore, the first example using “that” is the correct one, but many people would not consider the second ungrammatical.

It is, however, incorrect even in informal contexts to use that for a non-restrictive or parenthetical clause. For example, these sentences would be considered incorrect:

• This computer, that I have never liked, is very slow.
• The blue desk, that my father gave me.

An easy way to watch out for these is to look for instances where you have a comma followed by the word that. If I’d know this years ago, it would have saved me a lot of frustration with Microsoft Word!
Even though the usage of which has been relaxed to some extent, it is still better to keep your writing as clear as possible by using which for only non-restrictive clauses, and that for restrictive ones.
So, to return to Justin’s example:

• “To our knowledge, it is the only body in the solar system which currently sustains life, although several other bodies are under investigation.”
• “To our knowledge, it is the only body in the solar system that currently sustains life, although several other bodies are under investigation.”

The second sentence, using that is correct, but many people would consider the first sentence permissible too. In a formal context such as a scientific paper, it is better to use that for total clarity.


_from English Grammar 101_