Set off by comma

Question

My textbook says,SamData's graphics program SketchIt is easy to learn to use.....The reader assumes that SamData has more than one graphics program, and SketchIt tells which, so SketchIt is essential to the meaning of the main clause.If SketchIt is set off by two commas,(SamData's graphics program, SketchIt, is easy to learn to use.)the reader will assume that SamData has only one graphics program, which does not need to be named could be left out.For the underlined part, why is so? Do you have any grammar rule?

CalifJim · Accepted Answer

Supercat
My textbook says,
SamData's graphics program SketchIt is easy to learn to use.

....The reader assumes that SamData has more than one graphics program, and SketchIt tells which, so SketchIt is essential to the meaning of the main clause.
If SketchIt is set off by two commas,
(SamData's graphics program, SketchIt, is easy to learn to use.)
the reader will assume that SamData has only one graphics program, which does not need to be named could be left out.
For the underlined part, why is it so? Do you have any grammar rule?

This is the rule: Defining information is not set off by commas; non-defining information is set off by commas.

Defining information is the kind of information that helps the reader narrow down what the writer is talking about. Non-defining information is extra information that is added even though the reader already knows what the writer is talking about.

The cat which is black likes tuna. which is black is not set off by commas, so it's defining information. The reader needs to know that the cat is black before he or she can figure out which cat likes tuna. This suggests that there is more than one cat somewhere in the picture, and only one of them is black. That way which is black makes sense as defining information.

The cat, which is black, likes tuna. which is black is set off by commas, so it's non-defining information. The reader already knows that the cat is black. He or she does not need the information that the cat is black in order to figure out which cat the writer is talking about. This suggests that there is only one cat in the picture, and it's a black cat. That way which is black makes sense as non-defining information.

This rule applies mostly to relative clauses such as those in my examples above. In your post you have an example where the rule applies to nouns in apposition. In my experience authors are not so careful in this second case, so you may find situations where the rule is not followed for nouns in apposition.

CJ

Set off by comma

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