THE COMMA

A comma is used in the following situations:

           1. To separate items in a series. Use a comma to separate three or more items in a series. A comma should be inserted before the last conjunction (usually ‘and’ or ‘or’) in the sentence.

? Example: In the forest there were lions, tigers, and bears.

? Example: Participants in the study were able to read, play tennis, sleep, or watch television.

? Example: To write well you must read and take careful notes, use a pre-write strategy to generate ideas, and start your project early.

           2. To separate a nonessential clause. As described in the last chapter, a clause is a part of a sentence that contains a subject (or noun) and a predicate (verb or action). A nonessential clause is used like a theatrical aside or a by-the-way idea. The basic meaning of the sentence would still be intact without its inclusion. For example, the sentence below indicates the pie tasted stale. It just happened (by the way) to be made by Marvin.  

? Nonessential clause: The pie, which was made by Marvin, tasted stale.

? Without the clause: The pie tasted stale.

           The sentence above does not indicate that there are other pies; simply that the existing pie tasted stale. The fact that Marvin made the pie is an interesting aside but not an essential part of this sentence.

           Conversely, an essential clause in the middle of a sentence is vital to the stuff and the end and should not be separated by a comma.  The sentence below tells us that there was more than one pie, but only the pie that was made by Marvin tasted state. This is essential information, thus it is an essential clause. As described in the last chapter, ‘that’ is used instead of ‘which’ and there are no commas.

? Essential clause: The pie that was made by Marvin tasted stale.

     Below is another example of sentence with a nonessential clause.

? Nonessential clause: The study, which was conducted in 1968, showed significant results.

? Without the clause: The study showed significant results.

           This next sentence is also an example of a by-the-way clause. We are not comparing these students to other students. Instead, we are saying that they produced excellent papers; and by the way, they just happened to have started their projects early.

? Nonessential clause: The students, who started their projects early, produced excellent papers.

? Without the clause: The students produced excellent papers.

           3. To separate two independent clauses in a compound sentence. A compound sentence is one that has two or more clauses each of which makes sense by itself

? Not a compound sentence: On Tuesday I will be working at home.

? Compound sentence: It is Tuesday, and I will be working at home.

? Clause #1: It is Tuesday.

? Clause #2: I will be working at home.

           In the first sentence above, ‘On Tuesday’ is not a complete idea; however, ‘It is Tuesday’ is a complete idea. Use a comma followed by a conjunction to separate these ideas. The rule is this: If you have a compound sentence where both sides make sense as a stand-alone sentence, use a comma followed by a conjunction (and, but, or, yet, so, for, or not). 

           In the sentence below both parts of it would work as complete sentences. A comma/conjunction is used to separate them.

? Compound sentence: The tests were completed, and the books were put away.

? Clause #1: The tests were completed.

? Clause #2: The books were put away.

           Again, the sentence below shows that each clause is a separate idea and makes sense as a stand-alone sentence.

? Two independent clauses: There were many games being played, but poker seemed to be preferred by most.

? Clause #1: There were many games being played.

? Clause #2: Poker seems to be preferred by most.

           In the first sentence below, both clauses do not make sense as stand-alone sentences.

? Incorrect: John played professional baseball, and later played in a band. 

? Complete idea: John played baseball professional baseball. 

? Not a complete idea: Later played in a band.

? Correct: John played professional baseball and later played in a band.

     I know this gets confusing, but the insertion of the word ‘he’ in the second part of that same sentence changes things because the clause now becomes a complete idea.

? Correct: John played professional baseball, and he later played in a band.

? Complete idea: John played baseball professional baseball.

? Complete idea: He later played in a band

           4. After an introductory phrase. An introductory phrase sets up the next part of the sentence but does not make sense by itself. Use a comma to set this apart.

? After the test, John went home.

? Because it is Wednesday, the books were given to the students.

? As a result of the treatment, all participants showed improvement.

? If the apparatus works, the test can continue.

? If you have a compound sentence where both sides make sense as a stand-alone sentence, use a comma followed by a conjunction

This is an excerpt from my book: Academic Writing: Process and Product