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Apposition: 

When we use two noun phrases (np) next to each other in a clause, and they refer to the same person or thing, we call this apposition:

[NP 1]The living room, [NP 2] the biggest room in the house, looks out on to a beautiful garden. (The living room and the biggest room in the house are the same room.)

[NP 1]Timothy, [NP 2 ] their youngest child, is very musical. (Timothy and their youngest child are the same person.)

The second noun phrase tells us something more about the first noun phrase (its identity or its qualities). We can also reverse the order of the phrases:

[NP 1]The biggest room in the house, [NP 2] the living room, looks out on to a beautiful garden.

[NP 1]Their youngest child, [NP 2] Timothy, is very musical.

 

Types of apposition

In writing, we often separate the noun phrases by commas. We do this when the second noun phrase gives extra information which is not necessary to identify the person or thing:

Edinburgh, Scotland’s capital city, has a population of around 450,000. (Scotland’s capital city is extra information which is not necessary to identify Edinburgh.)

Sometimes the second noun phrase contains information which specifies which person or thing we are referring to from a number of possible people or things. In these cases, we don’t use a comma.

Compare

commas	no commas
My brother, Philip, works at the local museum. (The speaker probably only has one brother.)	My brother Mark is a police officer. My brother Joe is still at university. (The speaker has more than one brother. Mark and Joe specify which brother we are talking about.)