Commas: where to use them

Commas are not like salt. They can't be sprinkled over prose to give it flavour. For the most part, their presence is an indication of a pause in speech. For instance, they appear before or after adverb phrases, like this. Christopher Altman, of Onondaga Community College in Syracuse, New York, explains this in more detail here. (By the way, did you notice how the commas were used in the previous sentence to set off the city and state?

As well as being used to set off adverbial sentence modifiers, commas are used to separate items in a list. To illustrate, a comma has been used after the opening phrase of this sentence, as well as after each item in the following list of parts of speech: nouns, verbs, adjectives, and adverbs.

By the way, that last comma, the one before the phrase "and adverbs," is called the Oxford or serial comma. It's the one that comes just before the word and at the end of a list. As seen in the previous sentence, and the following one too, commas also set off appositive constructions. June, my teacher, loves commas.

Finally, commas are used to separate subordinate clauses from main clauses. This is illustrated by the following sentence. When Tom got home, Mary had already made dinner. The same punctuation pattern can be applied with "Whereas" or "As soon as" at the beginning of the sentence.

While we're on the subject of clauses, I should raise the thorny problem of restrictive versus non-restrictive clauses. As the name suggests, a restrictive clause (beginning with who or which) which restricts the meaning of a noun, and is therefore essential to identifying the subject, is not set off from the rest of the sentence by commas. Here is an example. The man who held the door open for you was the robber. (Without the who clause we don't know which man.)

On the other hand, a non-restrictive clause adds information that is additional, rather than essential to identifying the subject. Tom, who bought a used bicycle, asked me to go cycling with him. (This clause is non-restrictive, and therefore set off by commas. We do not need the information in the clause in order to identify Tom, who has already named in the sentence.)

Another role that commas play is to separate the main part of a sentence from quotations that are part of the same sentence, as follows. "John," she said, "Please come here." Here's another example: "Please come here, Sally," said John.

This brings us to one more use of commas. They are used to set off the name of the person being addressed directly in imperative sentences. "Bill, stop that." By the way, did you notice the second comma in the previous sentence? It sets off a phrase in the middle of the sentence, which in this case, happens to be a question.

One more use of commas is that they follow certain sentence connectors. For instance, we may use the word "however" to connect two main clauses, as in the following example. Tom hurried along the street when he saw the bus; however, it left before he got there. In the previous sentence, a semi-colon (more about those in a later post) is used before the sentence connector, and a comma after it. This punctuation pattern also applies to other sentence connectors, including "nevertheless" and "still."

If I had wanted to, I could have put an Oxford comma after "nevertheless" in the previous sentence. But since that is an optional comma, I chose not to.