The Clear Which Project
Halloween is only one day. But all year long, people have trouble with "whiches."
When should you use "which" and when "that"? And are commas necessary to separate the clause from the rest of the sentence?
To illustrate the rules, here are two similar instructions:
- Open the door that is blue.
- Open the door, which is blue.
The first sentence describes a situation with more than one door. To follow the instruction correctly, it's necessary for you to know the color of the door you should open. The color clause isn't set off with commas, because it's an integral part of the sentence.
The second sentence describes a situation with one door. You don't have to make a choice, so you don't need to know the door's color to complete the task. Since this color clause represents a slight departure from the main point, it is set off with commas.
Rule of Thumb (whiches' thumb, that is!)
Sometimes there are clauses that are required to make the sentence meaningful. Sometimes there are extra descriptive clauses, which aren't necessary to make the point.
And remember: a comma is the only thing "protecting" you from a which!