Parallel structure means expressing coordinated elements — items in a list, compared alternatives, or paired constructions — in the same grammatical form. "She likes hiking, swimming, and biking" is parallel (three gerunds); "She likes hiking, to swim, and bikes" is not. Parallelism applies to words, phrases, clauses, and even sentences in a series. Correlative conjunctions (both...and, not only...but also, either...or) impose especially strict parallelism requirements, because the element after the first conjunction must match the element after the second in grammatical form.
Underline the coordinated elements in sentences and check whether they share the same grammatical form — all nouns, all infinitives, all prepositional phrases. When they do not match, rewrite to make them parallel. Then practice constructing original sentences using correlative conjunctions, verifying that both sides mirror each other.
You already know from sentence structure that sentences combine words into functional units — subjects, verbs, objects, modifiers — and that coordinating conjunctions like *and*, *but*, and *or* join elements of equal grammatical rank. Parallel structure is the rule that governs what "equal rank" means in practice: when elements are joined by a conjunction, they must be expressed in the same grammatical form.
The most intuitive case is a simple list. *She enjoys hiking, swimming, and cycling* is parallel because all three items are gerunds (noun-like verb forms ending in *-ing*). Change one and the sentence breaks: *She enjoys hiking, to swim, and cycling.* The meaning is identical, but the form is inconsistent — two gerunds and an infinitive. Your ear already detects this as wrong, even if you can't name the rule. The fix is always to pick one form and apply it consistently: either *hiking, swimming, and cycling* or *to hike, to swim, and to cycle*.
The same principle extends to comparisons and compound predicates, where the elements on either side of the conjunction must match. *The new policy saves time and money* is parallel (two nouns after the verb). *The new policy saves time and is cheaper* is not — one noun and one predicate. To fix it: *The new policy saves time and costs less* (two verbs), or *The new policy is faster and cheaper* (two adjectives). You're not changing what's being said — you're choosing a consistent grammatical lane and staying in it.
Correlative conjunctions — paired conjunctions like *both…and*, *not only…but also*, *either…or*, *neither…nor* — impose the strictest parallelism requirement. Whatever grammatical form immediately follows the first conjunction must be mirrored exactly after the second. *She is both a doctor and a lawyer* is parallel (noun after *both*, noun after *and*). *She both is a doctor and practices law* is not — the *both* is placed before the verb, but *and* is placed before the noun phrase, so the two halves don't match. The fix: *She is both a doctor and a lawyer* (move *both* so both halves are noun phrases) or *She both treats patients and argues cases* (both halves are verb phrases).
To edit for parallelism, identify the conjunction, underline everything it connects, and ask: are all these items the same grammatical type — all nouns, all verbs, all adjectives, all prepositional phrases? If not, choose the form that fits best and rewrite the outliers. The goal is never to change the meaning — it is to express the same ideas in grammatically matching structures, making the sentence easier to process and more satisfying to read.