An appositive noun or noun phrase follows another noun or noun phrase in apposition to it; that is, it provides information that further identifies or defines it. Such “bonus facts” are framed by commas unless the appositive is restrictive (i.e., provides essential information about the noun).
Appose is a very old word that one doesn’t cross paths with much except in the realms of grammar and science. It came to English from Middle French via Latin and means “to put near, side by side, or alongside.” It doesn’t quite mean the same as oppose, but it is quite close in meaning to juxtapose. Apposition is used in medical science to describe how cell walls defend themselves by thickening themselves with layers of protection. In grammar, an element is said to be placed in apposition to another element if it provides an extra layer of description to it.
What is an appositive?
At its heart, an appositive is bonus information.
The core of this sentence is Hermione Granger is accomplished at spells. A witch at Hogwarts School is an appositive noun phrase that gives us additional information about Hermione Granger.
This is a sentence about where the Eiffel Tower can be found. The appositive phrase Gustave Eiffel’s masterpiece tells us a bit more about the sentence’s subject noun, Eiffel Tower.
Here, the core sentence is My childhood friend loved horses. It works as a sentence on its own, but the appositive, the proper noun Anne-Marie, gives the reader supplemental information about my friend. It renames her.
Commas and appositives
Appositive nouns and noun phrases are often nonrestrictive; that is, they can be omitted from a sentence without obscuring the identity of the nouns they describe. Another word for nonrestrictive is nonessential. Always bookend a nonrestrictive, appositive noun or phrase with commas in the middle of a sentence. If the noun or phrase is placed at the end of a sentence, it should be preceded by a comma.
Use commas to frame nonrestrictive elements
Depending on the tone you want to achieve and the context, you may also choose either parentheses or an em dash to frame a nonrestrictive appositive phrase.
Whichever way you choose to punctuate it, the key is to realize that My brother likens himself to Zeus is the core sentence and that the god of thunder is nonessential to that sentence. It is nice to know, but it is not essential in terms of function.
Think of a sentence with a nonrestrictive appositive in it as a motorcycle with a sidecar attached to it. The sidecar is a lovely addition to the motorcycle and changes the overall experience of taking it for a spin, but the motorcycle could go on without it. The nonrestrictive appositive is your sidecar, and it needs punctuation to attach it to the motorcycle, which is your sentence.
Commas and restrictive elements don’t mix
When an appositive noun or noun phrase contains an essential element without which a sentence’s meaning would materially alter, do not frame it with commas.
There are no commas here because Bill is an essential description of my friend. We can assume from this sentence that the speaker has many friends, but the one who owes them money is Bill. The unlikely circumstance under which the first sentence could be construed as correct would be if the speaker has only one confirmed friend, and that friend’s name is Bill.
Now think of a motorcycle again, except now without the sidecar. This is the restrictive appositive motorcycle. If anyone wants to hitch a ride on this motorcycle, they will have to ride double behind the driver. With this type of appositive, there is no disconnection between the driver and the passenger; they have their arms around each other. The restrictive appositive motorcycle zooms out of sight—without commas.