Which is that?

One of the commonest areas of confusion in English grammar regards the use of "which" and "that":

They enjoyed looking at the photos, which had been taken in Tokyo.

vs

They enjoyed looking at the photos that had been taken in Tokyo.


These words are used to connect two relative clauses in a sentence. Their correct use depends on whether the clause is restrictive or non-restrictive.

Use "that" when a clause is restrictive and limits the subject

Restrictive means that the clause limits the sentence to discussing one particular subset of a larger group - it literally restricts the sentence.  For example, "I enjoy movies that have large amounts of graphic nudity", or "They complained about the walls that had been poorly painted".

Here I am being specific. The movies I enjoy are restricted to those that contain graphic nudity;  the walls that they complained about were only - and so, restricted to - the ones that had been poorly painted.

Use "which" when a clause is non-restrictive and adds extra information to the subject

Non-restrictive means that additional information is being offered, and usually means that the second clause can be left out without altering the meaning of the sentence.

Examples are, "I always use an external monitor, which allows me to edit my photos more easily", or "For her exercises she uses a trampoline, which allows her to jump higher".

Comparison sentences

I always take my mobile phone that allows me to play games.
I always take my mobile phone, which allows me to play games.

In the first sentence the restrictive use of "that" suggests I have a choice of mobile phones to take with me, but I always take the one that has games on it.  In the second sentence the non-restrictive "which" shows that I always take my mobile phone, and - as an extra piece of information - it so happens that it allows me to play games on it.

Going back to the original discussion of photographs from Japan:



They enjoyed looking at the photos that had been taken in Tokyo.



They enjoyed looking at the photos, which had been taken in Tokyo.

Whoever 'they' are, in the first instance they only enjoyed the (restricted) set of photos from Tokyo.  Perhaps they thought the photos from Kyoto and Osaka were poorly composed.  The second sentence suggests that they enjoyed looking at all the photos - and as an additional piece of information, the photos happen to have been taken in Tokyo.