There are two types of relative clause:

Defining: gives essential information about the noun it relates to, making it clear what the noun refers to.
Many people of all ages enjoy TV commercials which rely on humour.
This means something different from Many people of all ages enjoy TV commercials.

Non-defining: gives additional but non-essential information. If this information is omitted, it is still clear what exactly the noun refers to.
In recent years, advertisers have had considerable success with viral marketing, which probably dates from 1996.

As the examples show, commas are used in non-defining clauses, but not in defining clauses. It is important to use punctuation correctly in relative clauses, as inaccurate use can change the meaning of the sentence.
A manufacturer of sports clothes has cancelled its advertisements which have been criticised by the public.
A manufacturer of sports clothes has cancelled its advertisements, which have been criticised by the public.
In the first example, only the advertisements which have been criticised have been cancelled, and the manufacturer’s other advertisements will continue. In the second example, all the advertisements have been cancelled because they have all been criticised.

Relative pronouns
In defining relative clauses, you can use:
- who or that when referring to people
I’m going to a talk to be given by an athlete who/that appeared in a TV commercial for running shoes.
- which or that when referring to things
The event which/that Toby Young describes was intended to advertise his latest book.

The relative pronoun can be left out when it is the object of a clause, as in the second example above, where it is what Toby Young described. It must be included when it is the subject, as in who/that appeared in a TV commercial for running shoes.

In non-defining relative clauses, you can use:
- who when referring to people
Gary Phillips, who set up his advertising agency ten years ago, has become very successful.
- which when referring to things
At yesterday’s meeting, which was arranged to discuss the current campaign, the manufacturer agreed to extend their contract with the advertising agency.
- which when referring to a whole clause
In one particular commercial, a man washes his hands in car oil, which is intended to imply that the oil is also good for car engines.
Here which refers to the whole of the underlined clause.

whose
Whose is used to refer to people and – less often – things.
Our next speaker is Sharon Cooper, whose book on the history of advertising was published last year.
A commercial whose strength lies in humour is more likely to be remembered than a more straightforward one.

whom
Whom is mostly used in fairly formal language. It can be the object in a clause, or follow a preposition.
The group whom the government wishes to protect from the effects of advertising is children.
To whom it may concern: Stephanie Allen has been employed at this advertising agency since September 2003.

where, when, why
These words can be used instead of a relative pronoun after appropriate nouns. It is possible to omit when and why in defining relative clauses, as in these examples:
The launch of a new product is the time (when) you see if all your hard work has been successful.
The brilliant advertising campaign for these running shoes is the reason (why) sales have rocketed.


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