Before we start this topic, let me mention which pronouns are used. They are the following: who, where, which. These clauses give extra information to the subject. They say something more about a person or a thing, something additional. This additional information is not necessary though; even without this additional information, we are still aware of what the subject is.

It is important to note, however, that the additional information is separated by two commas: in front of the clause and at the end of the clause.

Mr. Brown, who is our geography teacher, is 60 years old

London, which is the capital of the UK, has 5 million inhabitants

In both cases, we would know who or what these subjects are. The extra information separated by commas is nothing else but completions. They say something more about those subjects. With commas, it is easier to distinguish the clause from the main clause.

If we remove the non-defining clause, we still have a grammatically correct sentence:

Mr. Brown is 60 years old

London has 5 million inhabitants

As we see, both constructions are correct sentences. Non-defining clauses can be considered completions.

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