Using the comma
The comma we have today came from the virgule (/) used in the 13th to 17th centuries to indicate a pause in our reading. Since then, we have developed roughly nine specific uses of commas, and these usages can even influence the clarity and meaning of the sentence.
Some of these include:
- Before coordinating conjunctions joining independent clauses
- After an introductory clause or phrase
- Between (all) items in a series
- Between coordinated adjectives
- To set off non-restrictive clauses or phrases
a. Before coordinating conjunctions joining independent clauses
An independent clause is a group of words that contains a subject and verb and expresses a complete thought. We can join two independent clauses using a coordinating conjunction; however, we need to precede the conjunction with a comma. The coordinating conjunctions are captured with the acronym FANBOYS (for, and, nor, but, or, yet, so).
Jim studied in the pub for his chemistry quiz, but it was hard to concentrate because of the noise.
Each of the clauses in this sentence: “Jim studied in the pub for his chemistry quiz” and “it was hard to concentrate because of the noise”, express complete ideas. Here, they are joined with the coordinating conjunction “but.”
If we forget the coordinating conjunction, we create a comma splice.
Jim studied in the pub for his chemistry quiz, it was hard to concentrate because of the noise.
To correct a comma splice we can either add a coordinating conjunction after the comma or replace the comma with a semicolon.
Note that if the two independent clauses are short, we can omit the comma.
b. After an introductory clause or phrase
We usually separate an introductory clause or phrase from the rest of a sentence using a comma.
After the hospital had completed its fund-raising campaign, an anonymous donor contributed an additional $10,000.
In this sentence, the introductory phrase: “After the hospital had completed its fund-raising campaign” modifies the verb “contributed”, so we call it an adverb clause.
Note that if the introductory phrase is short, we can omit the comma.
c. Between items in a series
We use the comma to separate the items in a series when there are more than two items.
Playing in a band can be exciting, but many people do not realize the hardships involved: constant rehearsals, playing until 2 a.m., transporting heavy equipment to and from gigs, handling drunken audience members, and avoiding groupies.
Although William Strunk Jr. and E. B. White suggest in their book Elements of Style that we can skip the last comma before the conjunction, the best practice is to include it.
d. Between coordinated adjectives
This comma rule comes down to the difference between two kinds of adjectives: coordinated adjectives and cumulative adjectives.
Coordinated adjectives are adjectives in a row that each modify the noun. For example, in the phrase “heavy, bulky box,” both “heavy” and “bulky” modify the noun “box.” We need a comma to separate these.
Cumulative adjectives are adjectives that progressively modify the noun. The adjective closest to the noun modifies the noun and creates a new unit that the next adjective modifies. For example, in the phrase “exquisite custom truck,” “custom” modifies truck to create a new unit “custom truck”. Then “exquisite” modifies this new unit. We don’t use commas to separate cumulative adjectives.
If you’re not sure which kind of adjectives you have, try inserting the word and between them. If the phrase makes sense, then the adjectives are coordinated, if it doesn’t, or its meaning changes, then the adjectives are cumulative. Compare “heavy and bulky box” with “exquisite and custom truck.”
e. To set off non-restrictive clauses or phrases
Recall our discussion on choosing that versus which versus who.