Grammatically Correct: The writer's essential guide to punctuation ...

PUNCTUATION
SETTING OFF PARENTHETICAL ELEMENTS
What's the difference between the following two sentences?
Jack thought Melvin should have his head examined.
Jack, thought Melvin, should have his head examined.
In case A, someone named Jack is thinking about someone named
Melvin; in case B, Melvin is thinking about Jack.
\Vhat would happen: if we removed the words thought Melvin?
We would be left with:
Jack should have his head examined.
In case A, the sentence is completely changed by the removal of
these words: It is no longer referring to the right head. Case B, ·
in contrast, may have lost something-the fact that Jack's headexamining
need is merely a matter of someone's opinion-but at
least it still refers to the same head. The words enclosed by commas
in case B are an example of a parenthetical element-text that is
not critical to the basic structure of the sentence. The defining
characteristic of a parenthetical element is that if it were removed,
the remainder of the sentence would still be grammatically and
semantically intact. Some information would be lost or diminished,
but the essential meaning would not be altered.
Parenthetical text may be set off from the rest of the sentence by
three types of punctuation marks: parentheses, dashes or commas.
Parentheses (discussed on page 144) are best used when the element
is a decided digression and its removal would have little or no effect
on meaning: that is, they de-emphasize. Dashes (discussed on page
154) serve to draw particular attention to the element, making it
stand out; that is, they emphasize. Commas should be used when
the element is an integral part of the sentence but does not call for
any special attention. Enclosed by commas, a parenthetical element
just works its way quietly into the sentence, without fanfare.
The remainder of this section describes various types of sentence
constructions involving parenthetical elements. It is important that
you clearly distinguish these from nonparenthetical elements, so
that you know both where commas should go and where they should
not go.
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