Commas are generally used to make a sentence clear and easier to read. These are the basic rules for using commas. Commas should never be overused.
Use commas to set off introductory phrases or clauses unless they are very short.
- Trying to finish the abstract, the student worked until three a.m.
- After several hours of conversation, they decided to spend the next semester in France.
- In order to characterize the physiological adaptations of an organism to factors in its environment, it is often useful to examine the response of that organism to changing environmental conditions.
- In Virgil's version of the story, the ancient Sibyl of Cumae leads Aeneas into the kingdom of the dead.
Before a coordinate conjunction
Use a comma before a coordinate conjunction--and, but, or, for, nor, so, yet--when it connects two complete sentences.
- He gave millions to charity, and he was left with nothing.
- Ralph and Louise gave millions to charity.
- H. nudus were maintained in 100% artificial seawater, and P. clarkii were maintained in conditioned tapwater for several days.
- Virgil does not tell us much about the prophetess, but he attests to her wisdom and foresight.
Try it out:
- The women picketed all morning for the ERA but they left at noon to demonstrate against nuclear power.
- The women picketed all morning for the ERA but left at noon to demonstrate against nuclear power.
Separating coordinate conjunctions
Use commas to separate coordinate adjectives.
Coordinate adjectives are adjectives that equally modify a noun or pronoun. If you can insert the word "and" between them without making the sentence awkward, then they are coordinate, and a comma should replace the "and."
- She drives a little blue car. (No commas are used here because we cannot insert an "and." We wouldn't say, "a little and blue car.")
- She drives a comfortable, quiet car. (Yes, we would use commas here because we could say, "quiet and comfortable car.")
Try it out:
- He had several bright Hawaiian shirts.
- Sandra has a keen perceptive mind.
- The persistent shoe salesman talked me into a pair of Nikes.
Items in a series
Use commas to set off items in a series. The series may be made up of single words or a whole group of words, but in order for it to be a series, it must contain at least three items.
- Her apartment is large, elegant, and expensive.
- Harry Wilson, Amanda Waters, and Jane Clark have been friends since childhood.
- As part of the test, they had to climb a seven foot wall, run five miles, and crawl through a drainage pipe.
- They were coded as follows: flakes of type O were small and yellow, flakes of type 1 were large and yellow, flakes of type 2 were very fine and brown.
- Joyce enables us to see the worth of the common man, the underdog, and the outsider.
Try it out:
- Data processing electrical engineering and accounting are popular professions.
- Environmentalists are concerned about air water and noise pollution.
Before and after interrupters
Use commas before and after interrupters. An interrupter is a word or group of words that adds meaning to a sentence.
- She was, however, on the other team.
- My sister, who is an artist, lives on Vashon Island.
- I did, however, find evidence for competition in this treatment, as target biomass decreased significantly with increased neighbor biomass in both the low nutrient and high nutrient treatments.
- Leopold Bloom, the novel's protagonist, is presented as the title's Ulysses, and is physically and metaphorically characterized with the virtues of a classical hero.
- Douglas, on the other hand, could not take such an objective approach.
(Notice how the commas change the meaning in the following examples.)
- The Eighteenth Century novels that display prejudice should be eliminated from the course.
- The Eighteenth Century novels, which display prejudice, should be eliminated from the course.
- The neighbors who have dogs that bark should move.
- The neighbors, who have dogs that bark, should move.
Do not overuse commas!