The comma is both the most frequently used and abused mark of punctuation. Because commas are so regularly encountered, it is especially easy to acquire a habit of using them too frequently. This practice can result in writing that is long-winded, complicated, or simply incoherent. When properly employed, however, the comma may serve as the writer's best ally in elucidating otherwise complicated prose. The comma should be used:
Before conjunctions joining independent clauses
Ex. 1: "I have seen all of Martin Scorsese's films, but Casino is by far my favorite."
(The comma prevents the sentence from being read as: I have seen all of Martin Scorsese's films but Casino . . . Without the comma, a reader may assume that you have seen all of Scorsese's films except for Casino.)
Ex. 2: "The children gazed through the window, and the teacher cast a disapproving frown over their heads."
(In this case, the comma prevents the sentence from being read as: The children gazed through the window and the teacher . . . The visual pause prevents the reader from wrongly assuming that the children gazed through both the window and the teacher)
Also note: Similar problems occur when "or" and "for" are concerned.
- Sometimes a semicolon should replace a comma in these situations if the sentence is already heavily punctuated. ("My mother asked me to buy eggs, milk, and bread; but she did not specify the brands that she preferred.")
- Sometimes a comma is not necessary if the two clauses of a sentence are short and closely related ("John arrived early and Mary came an hour later.")
Between adjacent parallel items
Ex. 1: "I got out of bed at seven, left the house at eight, and was in the office shortly after nine."
(The comma separates members of a series that might otherwise run together in an incoherent string of words: the comma underscores the independence and chronological sequence of the events described.)
Ex. 2: "He was able to get the children reading, writing, and listening to what he was attempting to teach them."
(Here, the comma separates members of a series that might otherwise run together. "He was able to get the children reading writing . . ." suggests that the children were never asked to write, only to read what others had written.)
Around parenthetical elements
Ex. 1: "I decided that my essay, as well as those of my peers, required serious revisions."
Ex. 2: "Hamlet, arguably the most existentialist of the plays we have read, is the best piece I have studied this semester."
In sequences where needed to prevent misreading
Ex. 1: "Once you know, the answer seems obvious." (Prevents the sentence from being read as: Once you know the answer . . .)
Ex. 2: "I thought that I had been chosen, and prepared for the consequence." (Prevents the sentence from being read as: I thought that I had been chosen and prepared . . .)
In Special Contexts
- In dates written in month-day-year order, use commas both before and after the year: "She arrived on November 12, 1978, to inform us . . ."
- In addresses incorporated into the text, put commas after the addressee, the street, and the city: "Please forward my mail to Joseph Wilson, 213 Flag St., La Holla, CA 93124."
- Use a comma to indicate an ellipsis (omission of words that are implicit to the reader) only if the structure will be unclear otherwise: "The first exercise required that we write a timed essay; the second, that we evaluate the organizational quality of it; and the third, that we write a new draft based on the comments of a peer writer."
- Around a title or an affiliation that follows a name: "Sammy Davis, Jr."
- Separating the two parts of an idiomatic construction unless they are short and verbless: "The fewer people who work on this particular project, the better."
- Separating a direct quotation from the verb of saying that precedes or follows: "'You know that it isn't fair,' I replied."
Do NOT use commas:
- to separate correlative pairs: "This book not only addresses my field of interest(,) but the manner in which the author's perspective may be applied to numerous disciplines."
- to separate subject and verb: "People who are not interested in what I have to say and who prefer to focus the conversation on themselves(,) are not much fun to be around." (Note: the temptation to use a comma in such contexts may be an indication that you've taken too long to reach the verb. In such cases it may be wise to make further revisions in order to clarify the subject-verb relationship.)
- after a short introductory phrase or clause: "By 1762(,) they had resolved to establish a settlement in the New World."
- around transitional adverbs: "I should(,) perhaps(, )have mentioned it before." (Note: it is always necessary to set off the word "however" when it is used as a conjunction. Compare "However, John feels we should take the risk" to "However John feels, we should take the risk." In this case the misuse of a comma could seriously obscure the meaning of the sentence.)
Decide where the use of a comma clarifies each sentence's meaning.
1. My aunt bought presents for everybody but Joe didn't give it a second thought.
2. In the beginning it had seemed like a good idea.
3. I finished the paper on time. I was very happy to have done it.
4. She had been instructed to write the proposal make the Xeroxes and distribute them to the committee members.
5. I found the film to be not only boring but poorly directed.
6. He had worked for years for his family believed in a strong work ethic.
(Check out these sources for complete and comprehensive explanations of the comma):
Cook, Claire Kehrwald. Line by Line: How to Improve Your Own Writing. Boston: Houghton Mifflin Company, 1985. 108-31.
Turabian, Kate L. A Manual for Writers of Terms Papers, Theses, and Dissertations. Chicago: University of Chicago Press, 1996. 52-6.