In composition there are often choices that depend more on the subjective response of the writer than on rigid, objective rules of grammar. So it is with the use of commas, parenthesis, and dashes to set off thoughts.
We have previously discussed restrictive and non-restrictive words that modify. If, for example, a phrase or clause is restrictive, its meaning is essential to the meaning of its sentence, and it should not be set off by punctuation, such as a comma. Illustration: The car which is green is the one we shall drive. If “which is green” is omitted, we have no way of knowing which of several cars we are to drive, and the clause must not be set off by any punctuation; the clause restricts the meaning of car by setting it apart from other nearby cars of different colors.
Not so, however, with non-restrictive words, phrases, and clauses. Consider: The writer — who never touched a drop himself — spent much of his time in bars gathering information. As contrasted with The writer, who never touched a drop himself, spent much . . . or The writer (who never touched a drop himself) spent much . . . . Whether the writer drinks is not essential to the sentence’s thought, but the clause adds significant spice; and its message is probably most effectively delivered by using the dash.
Of the three, the comma is perhaps most subtle and sets the clause off most innocuously (quietly?); there are times when it works best, as in George, who is the oldest, is one of four children. The clause who is the oldest adds color but not drama, and the comma works well.
Parentheses are said to be digressive. They can set off words that are non-essential and almost irrelevant: The weather was treacherous, making it the worst possible night to be rushing to the hospital (is there ever a good time?). The thought within the parenthesis is somewhat off the subject but does add flavor and color to the sentence.
Note this about phrases and clauses which are set off by parentheses or dashes within sentences: even though the thought may be a complete sentence, it will begin with a lower case letter and will not take a period at the end, though it can take an exclamation point or question mark (as in the preceding paragraph). However, if the parenthetical thought is outside the sentence, it starts with a capital letter and is punctuated. Illustration: Sally has been violently ill for almost ten days. (This year’s flu epidemic has been the worst in years.) Note that with a parenthetical sentence which is outside a sentence all punctuation is inside the parentheses, and it would have been an error to put a period after the final parenthesis.
As is the case with so many tools of composition, proper use of the comma, parenthesis, and dash can make your prose dance. And in deciding which of the three to employ (my expressed opinions to the contrary notwithstanding!), there is a good bit of subjective latitude. Be consistent and have fun.
— Ken Butera