Parallel structure means that coordinate parts of a sentence, such as items in a series or list, have the same grammatical form. Items in a series must be all nouns, all verbs, or all participles, and so on. There are two reasons it is important to maintain parallelism in a series:
Sentences that maintain parallelism are much easier to read and process than those that do not. With parallel structure:
I like running, singing, and reading.
Without parallel structure:
I like running, singing, and to read In the parallel structure, all of the objects of “like” are gerunds, –ing verbs acting as nouns. In the non-parallel structure, there are two gerunds acting as nouns and one infinitive (to read), which makes the sentence awkward and harder to process.
Maintaining parallelism helps writers avoid grammatically incorrect sentences. Below is an example of a grammatically incorrect sentence without parallel structure:
The computer’s ability to multi-task, defend against viruses, and overall usability all improved when I updated the operating software.
The grammar error becomes clear when you make a bullet-point list of each item following the part of the sentence that each item is meant to complete:
The computer’s ability to:
“The computer’s ability to usability improved” is a grammatically incorrect sentence because all words following the phrase “the computer’s ability” must be verbs, but “usability” is a noun. By maintaining a parallel structure, you will avoid grammatically incorrect sentences containing lists.
If you are unsure if a sentence is parallel and has grammatically correct structure, try making a bullet-point list like the one above.
Writers should use parallel structure whether the items in the list or series are grammatical subjects, predicates, or objects.
Mowing the lawn, trimming the shrubs, and edging the sidewalk took her two hours to complete.
The writer interviewed two subject matter experts, edited four drafts, and composed two press releases.
On Saturdays they may visit the open-air market, the docks, or the park.