An appositive is a noun or a noun phrase placed next to another word to define or modify it. For example, in the phrase "my friend Leila," the noun "Leila" is an appositive.
An appositive can either be restrictive, or non-restrictive.
1. Non-restrictive appositives are not crucial to the meaning of the sentence. For example, in "Alan, the chief executive of the project, has just called for a meeting ", "the chief executive of the project" doesn't narrow down the first element "Alan". It rather provides additional information about it. Non-restrictive appositives are put between commas.
2. In a restrictive appositive, the second element limits or clarifies a word in some crucial way. For example, in the phrase "my friend Leila writes good poems ", the name " Leila" tells us which friend the speaker is talking about and is thus restrictive. Restrictive appositives don't need to be set off with a pair of commas.
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