Dictionary of English - while and whilst

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Which and that are both relative pronouns: they introduce relative clauses. The main difference between them is that while which can introduce both restrictive relative clauses: The zebra which the lion picked on was clearly sick, and non-restrictive relative clauses: The zebra, which lives in southern Africa, is related to the horse, that can introduce only restrictive relative clauses: The zebra that the lion picked on was clearly sick.

Which and that are equally acceptable in restrictive relative clauses; that is perhaps the less formal of the two.

Which can be used after a preposition: Is this the coat for which you paid 300?, but that cannot (you cannot say Is this the coat for that you paid 300? – although you can say Is this the coat that you paid 300 for?).

Which can refer to both nouns and pronouns: This is the one which I prefer and whole sentences: He's off sick, which is rather a shame, but that can refer only to nouns and pronouns: This is the one that I prefer.

When two separate relative clauses in the same sentence refer to the same noun, they should both be introduced by either which or that (or by who, if the noun refers to a person). Which is perhaps preferable to that in these parallel clauses: This is the system which Parsloe invented, and which has been used in the service for over twenty years. Don't mix which clauses with that clauses: This is the system that Parsloe invented, and which has been used in the service for over twenty years is not to be recommended.

Take care not to begin a relative clause with and which when there was no previous which clause for the and to relate to: This is the system invented by Parsloe, and which has been used in the service for over twenty years is not grammatical.