Commas - Appositives

We use a comma or commas to isolate an appositive—words or phrases that describe or clarify the noun that preceded it.  For example, “Einstein” is most famously the name of the exceptional physicist, but it is also less-famously the name of Dr. Emmett Brown’s dog from a high-grossing movie in the 80s, Back to the Future.

                                   Albert Einstein                                   

Einstein the dog

In the following sentence, there may be some confusion about which Einstein we are writing: 

Einstein was well known in the 80s. 

When we add more information to specify the Einstein to whom/which we refer, that information is isolated by commas: 

Einstein, one of the greatest thinkers in the history of humanity, was well known in the 80s.

Einstein, the dog from Back to the Future, was well known in the 80s. 

The extra information may be a single word or a long phrase.  Regardless of its length, the appositive is punctuated by a comma or commas.  If the appositive appears in the middle of the sentence, the appositive has commas surrounding it.  If the appositive concludes the sentence, it has a comma at its beginning and a period at its end:

Our family dog, Spot, lived to be fourteen years old.

My brother’s car, a red convertible, accelerates quickly.

Jennifer Smith, our family doctor, studied at McMaster University.

When Tracy was young, she lived in Upsala, a small town in Ontario.

Resources

Purdue OWL explains appositives well.

You can find an interactive quiz at ProProfs.