[Editor’s note: This post relies on the 16th edition of CMOS, which is no longer supported.]

Parallel Structure

Can you spy anything wrong with the following sentences?

1. I’ll either sign up Tuesday or Wednesday.
2. He looked in the trunk, the back seat, and under the hood.
3. They made snowballs, several snowmen, and skated on the pond.
4. I would buy cards, make a list of donors, and would write personal notes.

You probably guessed it: each sentence is blemished by a series of items that aren’t parallel in structure. In parallel structures, the main grammatical elements of all the items in the series match. 

Sentence 1

Sentence 1 is an either/or series. It is unbalanced because either is followed by a verb {sign up} while or is followed by a noun {Wednesday}. The structure will be parallel if both either and or introduce the same kind of element, whether it is a verb {I’ll either sign up Tuesday or do it Wednesday} or a noun {I’ll sign up either Tuesday or Wednesday}.

Sentence 2

In the series of prepositional phrases in sentence 2, two of the items have prepositions, but the middle one doesn’t. The structure can be made parallel by giving the middle item a preposition {in the trunk, under the back seat, and under the hood}. Of course, when all three items share a preposition, parallel syntax can be achieved more economically {e.g., in rain, fog, or snow}.

Sentence 3

Sentence 3 is meant to be a series of actions, but the middle item needs a verb to become parallel with the other two {They made snowballs, built several snowmen, and skated on the pond}.

Sentence 4

Sentence 4 is the most complex. Two of its items contain auxiliary verbs {would buy; would write}, but the third one doesn’t {make}. Giving the remaining item an auxiliary verb is a little clumsy {I would buy cards, [would] make a list of donors, and would write personal notes}. Somewhat better is to give all three items subjects and verbs {I would buy cards, I would make a list of donors, and I would write personal notes}.¹ Better yet, the first auxiliary can serve to introduce a series of three parallel verbs {I would buy cards, make a list of donors, and write personal notes}.

As you can see, there is sometimes more than one way to achieve parallel syntax. You can find more details and examples at CMOS 2.212–215.

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