What is the difference between

"The car, which is in the garage, needs a tuneup."

and

"The car that is in the gargage needs a tuneup."?

Answer: The first sentence implies that you own one car, or everyone already knows what car you are talking about. It needs a tuneup. The car happens to be in the garage.

The second sentence implies that you own more than one car. You are specifing what car you are referring to. It is the car that is in the garage which is in poor repair and needs a tuneup.

"Which" clauses are not essential to the understanding of the sentence. "That" clauses are. They clarify what specific object is meant. Notice that the "which" clause is set off by commas, and the "that" clause is not.

Let us assume that you have two samples that are undergoing different heat treatments to see what effect heating has on the sample's chemical reactivity. Sample A is put in the oven. Sample B is not.

1. The sample that is in the oven is Sample A.
2. Sample A, which is in the oven, is ready for testing.
3. Is "that" properly used in the limerick below?

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Created April 30, 1998, by Nancy Burnham and Fred Hutson.