Writing Tip: Proper Punctuation for Parenthetical Phrases

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Proper Punctuation for Parenthetical Phrases

by Mark Nichol

Writers often confuse a sentence that contains a parenthetical phrase starting with a conjunction with one that consists of two independent clauses divided by a conjunction, resulting in improper placement of punctuation. Here are a few examples, with discussion and revisions.

1. “The substance starts off in a higher energy state, and by combining with oxygen, ends up in a lower energy state.”
Remove the optional parenthetical phrase “and by combining with oxygen,” and the resulting grammatically flawed sentence is “The substance starts off in a higher energy state ends up in a lower energy state.” The conjunction andmust precede the first comma to produce the valid construction “The substance starts off in a higher energy state and ends up in a lower energy state”; therefore, the correctly punctuated revision is “The substance starts off in a higher energy state and, by combining with oxygen, ends up in a lower energy state.”

If the original sentence read, “The substance starts off in a higher energy state, and when it combines with oxygen, it ends up in a lower energy state,” it would be correct. Here, and begins an independent clause (“and when it combines with oxygen, it ends up in a lower energy state”) rather than preceding a parenthetical phrase (“by combining with oxygen”) that is followed by a resumption of the main clause (“ends up in a lower energy state”).

2. “Then the answer is given in more detail, with a fuller explanation, and where possible, some illuminating and fun trivia.”
When the parenthesis is omitted, the sentence that remains is “Then the answer is given in more detail, with a fuller explanation, some illuminating and fun trivia.” As in the previous example, the conjunction and is incorrectly thrown out with the rest of the phrase. (This construction also makes “with a fuller explanation” look like a parenthesis, too, rather than the beginning of a dependent clause.)

The parenthesis is “where possible,” not “and where possible,” so the second comma must follow, not precede, and: “Then the answer is given in more detail, with a fuller explanation and, where possible, some illuminating and fun trivia.”

3. “Deciduous trees decide to cut and run, or in other words, drop all their leaves at once.”
Other conjunctions can be misplaced, too: Here, or is mistakenly situated in the parenthetical phrase, resulting in the framing sentence “Deciduous trees decide to cut and run drop all their leaves at once.”

In this case, however, a comma is necessary before or as well as after it, even though the second half of the sentence is not an independent clause. The phrase “or, in other words, drop all their leaves at once” is an appositive — an elaboration that restates another word or phrase — to the informal descriptive phrase “cut and run,” so the proper revision is “Deciduous trees decide to cut and run, or, in other words, drop all their leaves at once.” (Without the parenthesis, a verbal nudge that the writer is using an amusing turn of phrase, the sentence reads, “Deciduous trees decide to cut and run, or drop all their leaves at once.”)