When Should I Put a Comma Before a Which Phrase?
Posted by Jake Magnum | October 16, 2017 | Proper Punctuation
One of the most common types of mistakes I see while proofreading involves commas and phrases beginning with the word which. Many writers seem to use a comma before which when it is not necessary or to forgo using a comma when one is needed.
A major reason for the commonality of this kind of mistake is that both ways can be grammatically correct. It is the context that determines which of the two are correct in any given circumstance. Another reason for this mistake is that, often, the difference between using a comma and not using one is very small.
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? Most people can’t, but there is a difference. 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 rule may seem completely arbitrary, but there is a reason for it. A comma before a which phrase signifies that the information after the word which is purely 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. If you simply said, “Go through the door,” the addressee would go through the red door, as it is their only choice. 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. 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 phrase is optional; a comma should not precede a which phrase when the information given in the phrase is crucial.