- Demonstrate the standard uses of commas as separators
Transition words add new viewpoints to your material; commas before and after transition words help to separate them from the sentence ideas they are describing. Transition words tend to appear at the beginning or in the middle of a sentence, as in the following:
- Therefore, the natural gas industry can only be understood fully through an analysis of these recent political changes.
- The lead prosecutor was prepared, however, for a situation like this.
When transition words appear between two complete ideas, however, a period or semicolon is required beforehand:
- Clint had been planning the trip with his kids for three months; however, when his boss called and asked him to work, he couldn’t say no.
- Sam was retired. Nevertheless, he wanted to help out.
As you can see from these examples, a comma is always required after a transition word.
Descriptive phrases often need to be separated from the things that they describe. Descriptive phrases tend to come at the very beginning of a sentence, right after the subject of a sentence, or at the very end of a sentence:
- Near the end of the eighteenth century, James Hutton introduced a point of view that radically changed scientists’ thinking about geologic processes.
- James Lovelock, who first measured CFCs globally, said in 1973 that CFCs constituted no conceivable hazard.
- All of the major industrialized nations approved, making the possibility a reality.
In each example, the phrase separated by the comma could be deleted from the sentence without destroying the sentence’s basic meaning. If the information is necessary to the primary sentence meaning, it should not be set off by commas. Let’s look at a quick example of this:
- Incorrect example: Juliet named her hamster after Zeus the god of thunder.
- Correct example: Juliet named her hamster after Zeus, the god of thunder.
The description of Zeus as the god of thunder is not necessary information for your audience to understand the meaning of the sentence, because Zeus’s identity is common knowledge. Therefore, the words the god of thunder should be set off with a comma.
However, sometimes this gets more tricky and there is more than one right answer, like in the sentences below:
- Jefferson’s son, Miles, just started college.
- Jefferson’s son Miles just started college.
Depending on the situation, either of these sentences could be correct. You would write the first sentence if Jefferson only has one son and his name is Miles. If Jefferson only has one son, then the name Miles is not needed information and should be set off with commas.
You would write the second sentence if Jefferson has multiple sons, and it is only his son Miles who just got into college. In the second sentence, Miles is necessary information, because until his name is stated, you can’t be sure which of Jefferson’s sons the sentence is talking about.
This test can be very helpful when you’re deciding whether or not to include commas in your writing.
Adjacent items are separated so that the reader can consider each item individually. Consider the following sentence:
- The river caught fire on July 4, 1968, in Cleveland, Ohio.
The dates (July 4, 1968) and places (Cleveland, Ohio,) are juxtaposed, and commas are needed because the juxtaposed items are clearly different from each other. This applies to countries as well as states: “Paris, France, is beautiful this time of year.”