Sentences can be simple. Or, by inserting a phrase within a sentence, as I’m doing here, they can become complex. Doing so by adding what’s called a parenthetical phrase, or a parenthetical, makes sentences richer and more informative; no one wants to read sentence after sentence at the level of complexity of “See Dick run.”
But writers must take care that when they surgically incise a sentence to insert a parenthetical, they suture the sentence at the right spot. Think of one comma as a hook holding the sentence open, and a second comma hooking it closed. (Dashes and parenthetical marks can be used, too, but this post focuses on the simplest and most common strategy.)
Consider this sentence: “As this process occurs, astronomers say the spectacle may even become a meteor storm.” It may seem fine at first, but notice that it appears to imply that the astronomers talk about the spectacle as the process occurs. That’s obviously not what it means.
“Astronomers say” is what’s called an attribution — identifying the source of a comment — and it’s often conveniently thrown into the middle of a sentence to provide this clarity. But if you insert such a parenthetical, you have to hitch the sentence open with one comma and close it back up with another: “As this process occurs, astronomers say, the spectacle may even become a meteor storm.”
Take a look at this sentence: “By 2030, demographers estimate twice as many people will live in urban areas as in rural regions.” Will the doubling occur that year, or will the demographers present their estimation at that time? It’s unclear, unless you signal that the reference to the demographers’ action is a parenthetical phrase, inserted into the root sentence to provide some context. “By 2030, demographers estimate, twice as many people will live in urban areas as in rural regions” accomplishes that goal.
Here’s another sentence ripe for misunderstanding: “Demonstrators rode models of the Segway Human Transporter, a scooter invented by Dean Kamen at a park Monday morning.” This sentence implies that the inventor conjured the idea at the park on Monday morning, and — voila! – the vehicles were being demonstrated days later.
Wrong. “Demonstrators rode models of the Segway Human Transporter, a scooter invented by Dean Kamen, at a park Monday morning.” (The phrase “a scooter invented by Dean Kamen” is a parenthetical dropped into the sentence “Demonstrators rode models of the Segway Human Transporter at a park Monday morning” to provide context.)
In the case of parentheticals, commas (or parentheses or dashes) work in pairs — but they have to cue up to the right location to do their job. When in doubt, test punctuation of parentheticals by temporarily removing the inserted phrase to determine whether the root sentence makes sense. If not, then the punctuation is misplaced.
For example, something is wrong in “They meet, and with collection permit in hand, head for the trails to gather seeds.” Omit the parenthetical, and the root sentence reads, “They meet head for the trails to gather seeds.” And must remain in the root sentence, so the first comma must follow, not precede, and.
Remember: For parentheticals, punctuation pals in pairs — and in the proper place.