When to Use a Comma Before a “Which” Phrase

When Should I Put a Comma Before a “Which” Phrase?

 
One of the most common mistakes I see when proofreading regards commas and “which phrases. Many writers seem to use a comma before the word “which” when it is not necessary to do so or  to forgo using a comma before the word “which” when one is needed.

A major reason for the commonality of this kind of mistake is that both using a comma and not using one are grammatically correct. It is the context that determines which of the two are correct. It also does not help that putting a comma before a “which” phrase changes the meaning of a sentence only very slightly.

To illustrate, read the two sentences that follow:

Go through the door which is red.

Go through the door, which is red.

Can you tell what the difference is between these two sentences in terms of their meaning?

The difference is this: In the first sentence (which has no comma), there is at least one other door besides the red one; in the second sentence (which has a comma), there is only one door, and it happens to be red.

This is because a comma before a “which” phrase signifies that the information in the “which” phrase is circumstantial. In other words, if the words after the comma were not in the sentence, it would not really make a difference.

If there is only one door which the addressee can possibly go through, it would not matter if you mentioned the color of the door. The instructions would be just as easy to follow without the “which” phrase. Thus, the information in the which phrase is circumstantial and, therefore, should be preceded by a comma.

If there are, say, three doors which the addressee could go through and you want them to go through the red one, it is important that the addressee is told the color of the door they are to go through. If the instructions read simply, “Go through the door,” the addressee would not know which door to go through and would become confused. Thus, the information in the “which” phrase is required and, therefore, should not be preceded by a comma.

To summarize, it is appropriate to use a comma before a “which” phrase when the information given in the “which” phrase is not completely necessary; a comma should not precede a “which” phrase when the information given in the “which” phrase is crucial.


 
Jake Magnum, author of the Magnum Proofreading Tips blog, is dedicated to helping writers perfect their work. In addition to giving free advice on his blog, Jake helps writers by offering very affordable proofreading services at magnumproofreading.com.

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