Writing tip 23: 'That' or 'which'?

Use 'that' to introduce a restrictive (essential/defining) statement. You cannot change a 'that' statement without changing the essential meaning of the sentence.

Example
The book that you need is unavailable.


Use 'which' to introduce a non-restrictive (non-essential/non-defining) statement. You can delete a 'which' statement and not alter the essential meaning of the sentence. A 'which' statement simply tells more about something.

Example
Hamilton, which is built on the banks of the Waikato River, has cold, foggy winters.

Note: 'Which' statements go inside commas, but 'that' statements do not.

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