The purpose of an appositive is to offer further details about the noun or pronoun that precedes it in a sentence. These sorts of phrases often arrive after a subject meant to be outfitted with more detail. However, appositives may also precede their associated nouns or pronouns.
Appositives Preceding Nouns
Nouns further developed by appositives may precede them in a sentence. For example:
"My sister Caroline is pregnant."
Here, the noun and subject of the sentence -- the sister -- is made more specific by the addition of an appositive, which, in this case, is her name: Caroline.
"Sheila's cat, an orange tabby, is adorable."
In this case, the appositive -- "an orange tabby" -- is more complex; instead of a single word like a name or a profession, we have a phrasal description of the preceding noun.
In these cases, the appositive almost interrupts the sentence; it does not appear at the sentence's opening, and there is more to say before the sentence reaches its close.
Appositives Following Nouns
Nouns may also follow appositives. For example:
"One of the most famous surrealist artists, Salvador Dalí often painted on huge canvases."
Here, Salvador Dalí is the essential subject of the sentence. However, the appositive or description arrives before his name is introduced.
When to Use Commas
Commas surround appositives that contain information nonessential or ancillary to the identity of the preceding noun. To wit:
"The provocative comedian Sam Kinison died in a car accident."
Here, the addition of the name -- Sam Kinison -- specifies who exactly the subject of the sentence -- the comedian -- is. The information is essential to his identity, and, therefore, no commas are required.
However, another version might read:
"Sam Kinison, a provocative comedian, died in a car accident."
In this case, the subject becomes Sam Kinison and, in turn, the commas surround the appositive: "a provocative comedian." In this sentence, we hope to draw focus to the car accident, not the man's occupation. The fact that he is a comedian becomes secondary, and the appositive is flanked by commas.
A dash or colon, as well as a comma, can be used to set off appositives.