We use appositives all the time; they’re the nouns or noun phrase we use for clarification. Basically, they rename a noun used before it to offer more information for the reader. Most of us know that instinctively, but a lot of the suggestions I make as an editor is about punctuation.
Now determining whether an appositive needs to be set off by a comma depends on whether the clause is restrictive or nonrestrictive. If it’s restrictive, it is essential to the meaning of the sentence and does not require a comma; conversely, if it’s nonrestrictive, then it could be taken out of the sentence without changing the meaning and should be set off by commas.
That’s a lot to take in all at once, so let’s go over some examples:
Jason’s dog, Sammy, loves to play fetch in the park.
Max’s sister, Catherine, is arriving later tonight.
Both of these examples have nonrestrictive appositives–Jason has one dog, Max has one sister–it’s all additional information and is required to be set off by commas.
Let’s take a look at a couple more:
Jason’s dog Sammy loves to play fetch in the park.
Anna has read Shakespeare’s famous play Hamlet at least ten times.
These appositives are restrictive–Jason has more than one dog, so “Sammy” is required to understand which dog we’re talking about and Shakespeare has written more than one famous play–no commas are needed in either example.
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