That or which

That or whichThe simple answer? That defines while which gives extra information.

Still confused? It’s all in the clauses.

Restrictive clauses

Use that to introduce restrictive clauses that add crucial information to the sentence.

The bag that I left on the train contained important papers.

When you’re adding a clause that contains essential information about the noun that comes before it, you’re being specific.

Rule of thumb: it’s incorrect to put commas or dashes around a restrictive clause, so if no comma is needed use that.

Test to decide: if you leave out the clause and the sentence doesn’t make sense you should be using that.

Non-restrictive clauses

Use which in clauses that give purely incidental information. A non-restrictive relative clause contains extra information that could be left out of the sentence without affecting its meaning or structure.

Non-restrictive clauses are enclosed in commas or dashes to indicate they’re dispensable from the sentence.

Hounslow – which is where Tom grew up – is a borough in West London.

The doctor was late for his appointment, which was not like him at all.

Rule of thumb: a comma to set off the extra information precedes a non-restrictive clause, so if a comma is needed use which.

The test to decide: if you completely remove the clause contained by the commas and the sentence still makes sense you should be using which.

Sometimes the difference is subtle as in these examples:

I was wearing the shoes that I bought to wear to the wedding. This is a restrictive clause. It's clear that the shoes were bought to wear to the wedding.

I bought a new pair of shoes, which I will wear to the wedding. This is a non-restrictive clause. The shoes were not specifically bought to wear to the wedding.

The dresses that are red are made of silk. This leads the reader to believe only the red dresses are made of silk.

The dresses, which are red, are made of silk. All the dresses are made of silk, and the fact that they are red is additional information.

Here the difference is clearer:

He started to write a poem that was terrible. (The poem was terrible.)

He started to write a poem, which was terrible. (The fact that he was writing a poem was terrible.)