Non-defining relative clauses

NON-DEFINING RELATIVE CLAUSES

The information in these clauses is not essential. It tells us more about someone or something, but it does not help us to identify them or it.
Compare:

Dogs that like cats are very unusual. (This tells us which dogs we are talking about).
Gorillas, which are large and orignate in Africa, can sometimes be found in zoos. (This gives us some extra information about gorillas - we are talking about all gorillas, not just one type or group).
John's mother, who lives in Scotland, has 6 grandchildren. (We know who John's mother is, and he only has one. The important information is the number of grandchildren, but the fact that she lives in Scotland might be followed with the words "by the way" - it is additional information).

Punctuation
Non-defining relative clauses are always separated from the rest of the sentence by commas. The commas have a similar function to brackets:

My friend John has just written a best-selling novel. (He went to the same school as me)
My friend John, who went to the same school as me, has just written a best-selling novel.

Relative pronouns in non-defining clauses

	Person	Thing	Place
Subject	who	which
Object	who/whom	which	where
Possessive	whose

Notes

In non-defining clauses, you cannot use 'that' instead of who, whom or which.
You cannot leave out the relative pronoun, even when it is the object of the verb in the relative clause:
He gave me the letter, which was in a blue envelope.
He gave me the letter, which I read immediately
The preposition in these clauses can go at the end of the clause, e.g.
This is Stratford-on-Avon, which you have all heard about.

This pattern is often used in spoken English, but in written or formal English you can also put the preposition before the pronoun: e.g. Stratford-on-Avon, about which many people have written is Shakespeare's birthplace.

Non-defining clauses can be introduced by expressions like all of, many of + relative pronoun:

	Person	Thing
*all of*	+ whom	+ which
*any of*	+ whom	+ which
*(a) few of*	+ whom	+ which
*both of*	+ whom	+ which
*each of*	+ whom	+ which
*either of*	+ whom	+ which
*half of*	+ whom	+ which
*many of*	+ whom	+ which
*most of*	+ whom	+ which
*much of*	+ whom	+ which
*none of*	+ whom	+ which
*one of*	+ whom	+ which
*two of etc...*	+ whom	+ which

Examples

There were a lot of people at the party, many of whom I had known for years.
He was carrying his belongings, many of which were broken.
The relative pronoun which at the beginning of a non-defining relative clause, can refer to all the information contained in the previous part of the sentence, rather than to just one word.
Chris did really well in his exams, which was a big surprise. (= the fact that he did well in his exams was a big surprise).
A socialist and a conservative agreed on the new law, which is most unusual. (= the fact that they agreed is unusual).