Use commas to set off short ideas, such as this one, that “interrupt” the main sentence.
[This is another in a series of grammar posts I'll be reprinting from my now defunct Suite 101 page. Comma Rule No. 1 can be found here.]
“Don’t interrupt!” That’s good advice when it comes from parents or teachers, but when composing a sentence, often interrupt the main idea to add an important secondary idea or for sentence variety. An interrupter can be a word, a phrase, or even a longer sentence that is inserted into main sentence. In this example,
The original members of the band—who were all born in Coventry—came together in the early ‘70s.
The phrase “who were all born in Coventry” is an interrupter. Notice that you can take it and the sentence still makes perfect sense.
An interrupter can be helpful for including a short piece of information that adds something to understanding of the subject, but which are not important enough to start a new sentence.
Sentence interrupters are a common feature of the English language; however, they need to be punctuated properly.
Short Words and Phrases
Use a comma on both sides of a short interrupting word or phrase:
You are welcome, of course, to come to dinner.
Novice writers often remember to put in the first comma but not the second:
You are welcome, of course to come to dinner.
The second comma, however, clarifies the phrase as an interrupter.
An appositive phrase is a special kind of interrupter that is used to identify the preceding noun:
John Smith, director of human resources, said that the company is hiring for several positions.
Jim brought his guitar, a 12-string Rickenbacker, to the party.
For long interrupting phrases or phrases with internal punctuation, it is common practice to use dashes instead of commas. Notice that dashes both precede and follow the interrupter, just as a comma would:
The Beatles—an English rock group consisting of John Lennon, Paul McCartney, George Harrison, and Ringo Star—impacted Western culture in significant ways.
Writers also use dashes instead of commas to draw attention to a particular idea:
You are welcome—of course—to come to dinner.
In this case, the writer wants to strongly emphasize the invitation.
Where are commas needed in the following sentence? (Answer appears at end of article.)
Learning grammar and punctuation like learning to play a fine instrument gives writers more power and flexibility in their writing.
Use commas to offset short interrupting ideas in a sentence, but be sure to place commas both before and after the interrupter. (Answer: Place a comma after “punctuation” and after “instrument.”)