Parallel structure, also called parallelism, means using the same grammatical structure for ideas that are similar or of equal importance. Basically, similar ideas should be in similar form. Parallel structure is most often used at the word, phrase, and clause levels. See the examples below.
Examples of Parallel Structure
Nonparallel
Yara loves running, to swim, and biking.
Parallel
Yara loves running, swimming, and biking.
The second sentence is easier to read than the second. The second sentence uses parallelism—all three verbs are gerunds, whereas in the first sentence two are gerunds and one is an infinitive. The first sentence is technically correct, but the mixed grammatical forms create a discord that distracts the reader. Using parallelism improves the style and readability.
Nonparallel
The dog ran across the yard, jumped over the fence, and down the alley sprinted.
Parallel
The dog ran across the yard, jumped over the fence, and sprinted down the alley.
In the first example, the third item in the series (down the alley sprinted) breaks the pattern of word order shared by the first two items. While reversing word order in surprising ways can have a literary effect, in many writing situations, it can be distracting. Always consider the writing situation when choosing to break expected writing patterns.
Nonparallel
We can pay with a mark, a yen, buck, or pound.
I went to the store on Monday, Wednesday, and on Friday.
Parallel
We can pay with a mark, a yen, a buck, or a pound.
I went to the store on Monday, Wednesday, and Friday.
Parallelism requires that articles (a, an, or the) and prepositions be used consistently when working with coordinate items or items in a list. For example, you can place a preposition or article in front of each item in a list (the good, the bad, and the ugly) or in front of the entire list itself (the good, bad, and ugly). It's a matter of preference and style; just be consistent.
Correlative Expressions
Correlative expressions are formed by using correlative conjunctions, such as both/and, not/but, not only/but also, either/or, and no sooner/than. Correlative expressions should use parallel structure as well. Here are some examples:
Nonparallel
Either you must grant her request or gain her disapproval.
Parallel
You must either grant her request or gain her disapproval.
Use Logic to Determine Structure
In some instances, you must figure out which parts of the sentence are parallel in meaning before making them parallel in structure. For example, take a look at the following sentence:
Sal applied himself in his new job, arriving early every day, skipping lunch regularly, and leaving late every night.
In the sentence above, the -ing participle phrases ("arriving early every day," "skipping lunch regularly," and "leaving late every night") are parallel. The verb phrase in the main clause—"applied himself in his new job"—is not parallel to these participle phrases. This is because the main verb is "applied." The -ing phrases simply provide additional information about how Sal applied himself. It would alter the meaning of the sentence to make all of these verbal phrases parallel:
Sal applied himself in his new job, arrived early every day, skipped lunch regularly, and left late every night.
This version gives all the activities equal emphasis, instead of making the last three activities subordinate to the main activity ("applied himself in his new job").
Use parallel structure to demonstrate control over your sentences and to provide a better reading experience for your audience.
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