Parallel Structure
Parallel structure means using the same grammatical form for items that share the same function in a sentence — matching verb forms in a list, matching phrase types in a comparison, matching clause structures joined by a conjunction — and breaking that match, even subtly, is one of the most common ways a technically grammatical sentence still ends up reading awkwardly.
The core principle: match the grammatical form
Items in a list or series should share the same grammatical form: She likes hiking, swimming, and biking (all gerunds) rather than She likes hiking, to swim, and biking (mixing a gerund with an infinitive). The content of each item can differ freely — hiking, swimming, and biking are three different activities — but their grammatical shape needs to match for the sentence to read smoothly and predictably.
Parallelism with paired conjunctions
Correlative conjunction pairs (either...or, neither...nor, not only...but also, both...and) require the same grammatical structure to follow each half of the pair: She not only finished the report but also presented it to the board (both halves are verb phrases) rather than She not only finished the report but also the presentation was given (mismatched — a verb phrase paired with a noun-plus-passive-clause structure), which forces the reader to mentally realign the sentence's structure partway through.
Parallelism in comparisons
Comparisons using 'than' or 'as' also require matching structures on both sides: Running is more enjoyable than to swim should be Running is more enjoyable than swimming, matching gerund to gerund rather than gerund to infinitive. The mismatch here isn't wrong in a way that prevents understanding, but it creates a small, persistent friction for the reader that a parallel version removes entirely.
Parallelism across longer, multi-item lists
The longer a list gets, the more a parallelism break stands out, since a reader builds an expectation from the first couple of items that later items then violate: The new policy aims to reduce costs, improving efficiency, and it will boost morale mixes an infinitive phrase, a gerund phrase, and a full independent clause across just three list items, forcing a much heavier structural adjustment than a properly parallel version: The new policy aims to reduce costs, improve efficiency, and boost morale (three matching infinitive verb phrases).
Why parallelism problems are easy to introduce during revision
Parallel-structure errors often creep in specifically during editing, when a writer adds or rewords one item in an existing list without checking that the new wording still matches the grammatical form of the surrounding items — a perfectly parallel sentence in an early draft can silently lose its parallelism after a single item gets revised, which is why a dedicated pass specifically checking list and comparison structures is worth doing on any heavily edited document.
Parallelism in headings, bullet points, and resumes
Parallel structure is especially important, and especially visible when broken, in bulleted lists, section headings, and résumé bullet points, where each item's grammatical form sits in close visual proximity to the others: Managed a team of ten, Led product launches, and Was responsible for budgeting breaks parallelism (two active-verb phrases followed by a passive-voice-flavored phrase), while Managed a team of ten, Led product launches, and Oversaw the annual budget restores it, matching all three bullets to the same active past-tense-verb pattern.
How to catch parallelism problems in your own writing
A reliable check is to read only the shared connecting words of a series or comparison (the verb stems, the conjunctions) in isolation from their surrounding content, confirming each item plugs into the same grammatical slot the same way. For a list, this means reading just the first word or two of each item back to back; for a correlative pair, it means confirming the same part of speech follows each half of the construction.
Parallelism and readability research
Studies of reading comprehension and processing speed generally support the intuitive sense that parallel structures are easier to read quickly and accurately than mismatched ones, since a reader's brain builds a predictive template from the first item in a series and processes each subsequent item faster when it matches that template — a mismatch forces additional processing effort exactly at the point in the sentence where a reader expects a quick, familiar pattern to continue.
Parallelism failures in official and legal documents
Poorly maintained parallel structure in legal and official documents can create genuine ambiguity about scope, not just an awkward reading experience: a clause listing prohibited activities using inconsistent grammatical forms may leave room to argue that one item in the list functions differently than the others, which is part of why professional legal drafting guides treat parallel structure as a substantive drafting concern rather than a purely cosmetic one.
Parallelism with negatives and exceptions
Parallel structure needs particular care when a sentence includes both an affirmative element and a negation or exception: The policy applies to all employees, not just management (parallel — both halves are noun phrases describing 'employees' and 'management') versus a mismatched version that pairs a noun phrase against a full clause, which tends to obscure exactly what the exception is meant to cover.
A worked example showing a full revision
Consider: The workshop will teach you to write clearly, effective communication, and how presentations are structured. Identifying the three list items' mismatched forms (an infinitive phrase, a noun phrase, and a full passive clause) makes the fix straightforward: The workshop will teach you to write clearly, communicate effectively, and structure presentations — three matching infinitive verb phrases, restoring both grammatical consistency and a smoother reading rhythm.
Parallelism across an entire paragraph, not just one sentence
The same underlying principle can extend beyond a single sentence to a series of related sentences within a paragraph, especially in instructional or step-by-step writing, where readers benefit from each step beginning with the same grammatical pattern: First, gather your materials. Next, measure the ingredients. Then, combine them slowly. Maintaining this sentence-initial parallelism across multiple sentences, not just within one list, makes multi-step instructions substantially easier to follow.
A final checklist for auditing parallelism in a finished draft
When reviewing a finished document specifically for parallel structure, scan for every list (marked by commas, bullet points, or 'and'/'or'), every correlative conjunction pair, and every comparison using 'than' or 'as,' and confirm that the grammatical form on each side or in each list position genuinely matches. This is a narrow, mechanical check, similar in spirit to auditing subject-verb agreement, and it's worth a dedicated pass separate from a general readability review.
Parallelism as a broader principle of clear writing
Beyond the mechanical rule of matching grammatical forms, parallel structure reflects a broader principle in clear writing: presenting equivalent ideas in equivalent forms signals to the reader that those ideas genuinely belong together at the same level of importance, while inconsistent forms can unintentionally suggest one item is somehow different in kind or weight from the others in the same list, even when that wasn't the writer's intention.
Parallelism in spoken rhetoric and memorable phrasing
Some of the most memorable lines in speeches and writing rely heavily on deliberate parallel structure for rhetorical power — phrases built from matched repeating grammatical patterns tend to stick in a listener's memory far more effectively than an equivalent idea expressed with mismatched, varied structures, which is part of why professional speechwriters treat parallelism as a rhetorical tool, not just a grammatical nicety. This is one reason parallelism is drilled so heavily in public speaking and persuasive writing instruction specifically.
Frequently asked questions
Is 'I like to run, swim, and biking' wrong?
Yes — this mixes an infinitive (to run), a bare verb after the shared 'to' (swim), and a gerund (biking) in the same list. A parallel version would use one consistent form throughout: I like running, swimming, and biking, or I like to run, swim, and bike. Small inconsistencies add up quickly across a long document if left unchecked.
Does parallel structure apply to single words as well as phrases?
Yes — the same principle applies whether list items are single words, phrases, or full clauses; whatever grammatical category the first item establishes, every following item in that same list or comparison should match it. A short list checked in isolation rarely takes more than a moment to fix once identified.
Why does broken parallelism feel awkward even though it's not always outright ungrammatical?
Because a reader unconsciously predicts each new list item will continue the grammatical pattern the earlier items established, and a mismatch forces an unexpected mental re-parsing mid-sentence, creating a small but real disruption to the sentence's flow even when the individual words themselves are all grammatically valid.
Does parallel structure apply to formal writing only?
No — it improves readability in any register, from casual writing to marketing copy to academic prose, though the consequences of breaking it (looking unpolished, creating ambiguity) tend to matter more in formal and professional writing than in casual conversation. Reviewing a draft specifically for parallel structure is a quick, high-value edit relative to how little time it typically takes to fix once spotted.