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‘Which’ or ‘that’ – or neither?

A recent post by professional editor Laura Hale Brockway deals with the differences between which and that. This is a tricky problem (especially if  English isn’t your first language), and she sets out to explain the ‘rules’ without resorting to linguistic jargon. But some of the advice she gives is contradicted by the evidence of usage. She says, for example, that which and that ‘are not interchangeable’, and that ‘they should certainly never be mixed for the sake of word variation’. But what about these two definitions, from different dictionaries, of the term carbon sink? The Macmillan Dictionary definition reads:

an ocean, forest, or other area of vegetation that helps to protect the environment by taking in large amounts of carbon dioxide from the atmosphere

But another learner’s dictionary (all right, it’s the Cambridge one) has this:

an area of forest which is large enough to absorb important and noticeable amounts of carbon dioxide from the Earth’s atmosphere and therefore to reduce the effect of global warming

It would be perfectly OK to switch these around, with that in the second definition and which in the first. On the question of punctuation, Brockway continues:

“Which” always mandates the use of a comma.

In other words, if you use which, you must have a comma before it. But surely it’s the other way around: if you need to use a comma, then you have to use which after it (not that), but – as the second definition above shows – there are plenty of situations where you can use which without a comma too.

In an attempt to resolve the confusion, we’ll try to answer three questions:

  • when are which and that interchangeable, and when do you have to use one or the other?
  • when do you need to use a comma?
  • when can you drop the relative pronoun altogether? (That’s the only bit of jargon we need: in the uses we’re discussing here, that and which are relative pronouns.)

The best place to start is with some sentences from our corpus. First, without a comma:

1. ‘Evergreen’ is a song which will always remind you of 2002.
2. There are a few books which I constantly re-read.
3. We will see the house which inspired Thackeray to write “Vanity Fair”.

Now, with a comma:

4. Each participant brought a song, which was added to an emerging co-created playlist.
5. The books, which cost about £4.50 each, are charged to customers’ mobile bills.
6. Ellen had no assets other than her house, which was valued at £200,000.

In 1,2,and 3, the relative clause (the part that begins with which) is essential because it tells you what is special or particular about the noun that comes before it. In 4,5, and 6, the relative clause provides additional information about the preceding noun, but the sentence would still make sense without it.  Saying, simply, The books are charged to customers’ mobile bills makes perfect sense – but saying ‘Evergreen’ is a song doesn’t communicate anything meaningful. In 1,2,and 3, which and that are interchangeable, but in 4,5,and 6 (where a comma is used) they are not.

The other case where a comma is needed (and therefore which should be used) is when the relative clause applies not to the preceding noun but to the whole preceding statement:

7. You had to go down the sloping floor of the rift to get there, which was a bit awkward.

Finally (an issue that wasn’t discussed in Brockway’s posting) there are cases where a relative clause doesn’t need either which or that. Compare:

8. ‘Evergreen’ is a song which/that will always remind you of 2002.
9. ‘Evergreen’ is a song which/that she always plays when she’s feeling sad.

In 9, you can drop the pronoun and say Evergreen is a song she always plays when she’s feeling sad. But you can’t do this with 8. Why? In 8, the song is the subject of the clause , but in 9 it’s the object (the subject is ‘she’) – and you can only drop the pronoun when the noun that follows is the object. (So you can also drop the relative pronoun in sentence 2, above, but not in 1 or 3.) This is also explained, by the way, in the entries for that (sense 7) and which (sense 2) in the Macmillan Dictionary.

If any of this isn’t clear, use a Comment to let us know. And if you have other questions of the same type, we’ll do our best to answer them.

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Michael Rundell


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