Commas around relative clauses

Another common problem in students' writing is the misuse of commas around relative clauses; that is, parts of the sentence beginning with 'which, that, who, whom, whose and where' that add meaning to a noun. For example:

Emperor penguins, which are one of the largest penguin species, make an annual mass trek in Antarctica.

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Whether or not you need to use commas depends on the intended meaning of the sentence. Sometimes it will be appropriate to surround a relative clause with commas; at other times, it will not. Look at and listen to the differences in meaning in each of the following sentences. This will give you some idea of when commas are needed and when they are not.

In example 1, by placing commas around the relative clause, the writer is indicating that this part of the sentence can be removed without changing the meaning of the sentence. The result is this sentence has the same meaning as:

'Books will be removed from the library (by the way, books are out of date)'.

The commas thus work almost like brackets, sectioning off the non-essential or 'non-limiting' information.

In example 2, by placing no commas around the relative clause, the writer is indicating that this part of the sentence CANNOT be removed without changing its meaning. In this example, the relative clause 'which are out of date' places a limit on which books will be removed from the library. In this example, only the out of date books will be removed from the library.

If we break up these two sentences into subject and verb, we can see that in one sentence only the word 'books' is the subject of the verb. In the other one, the relative clause is part of the subject as well.

From this example, you can see that commas play an important role in signalling meaning and structure.

When a relative clause provides ESSENTIAL information that places a limit on the sentence subject, NO commas are used.

When a relative clause adds NON-ESSENTIAL information to the sentence subject, COMMAS are placed around the clause.

Below are some more examples of sentences in which the commas around the relative clause change the message of the sentence.