Parallel structure means every item in a list, and every side of a comparison, is built from the same grammatical part: noun with noun, verb with verb, phrase with phrase. Break that match and a sentence doesn’t usually become wrong exactly — it just goes lopsided, and a reader feels the wobble before they can say why.
Here’s the classic broken version: “problems for both the winners and for those who lose.” One side is a noun (“the winners”), the other is a clause (“those who lose”). Match the parts and it snaps into place: “problems for both winners and losers.” Same meaning, half the effort to read.
The one-line test: read it as a list
Here’s the rule you can actually repeat to a labmate: find the shared stem, then check that every branch off it is the same part of speech. Take this sentence:
Wrong: “Mike learned to read poems critically and he appreciated good prose.”
The stem is “Mike learned to…” Plug in the first branch and it works: “Mike learned to read poems critically.” Plug in the second branch and it doesn’t — “Mike learned to he appreciated good prose” is nonsense, because the second half quietly switched from an infinitive to a full clause. Fix it by making both branches infinitives:
Right: “Mike learned to read poems critically and to appreciate good prose.”
That’s the whole test. You don’t need to name the parts of speech out loud — you just need to notice when plugging a branch back into the stem stops sounding like a sentence.
Where it hides in an aims sentence
This is the version we see constantly in the last line of an introduction, where a paper states its objectives:
Wrong: “The aim of this study was to characterize the mutation and evaluating its downstream effects.”
Run the test: the stem is “The aim … was to…” — “to characterize the mutation” plugs in fine, “to evaluating its downstream effects” doesn’t, because it drifted from infinitive to gerund halfway through. One word fixes it:
Right: “The aim of this study was to characterize the mutation and evaluate its downstream effects.”
It’s an easy slip to make, because both halves sound fine read alone — the mismatch only shows up once you say them against the shared “to.”
The version that hides in a comparison
Parallelism isn’t only about lists joined by “and” — it governs comparisons too, and this one is sneakier because the sentence still parses, it just compares two different kinds of thing:
Wrong: “Our results were more consistent than the control group.”
Read literally, that compares “our results” to “the control group” — a set of numbers against a group of people. What the writer means is a comparison between two sets of results:
Right: “Our results were more consistent than those of the control group.”
“Those of” is doing real work there — it’s what keeps both sides of “than” the same kind of thing. Reviewers rarely flag this explicitly, but it’s exactly the sort of line that makes a paragraph feel slightly harder to trust than it should, even when every fact in it is solid.
Why it’s worth catching before submission
None of these errors change what the data show. What they change is how much work a reader has to do to get there — and a reviewer who’s re-reading your aims sentence to figure out what you meant is a reviewer who’s no longer thinking about your results. Parallel structure is one of those things that costs nothing to fix and quietly buys back a reviewer’s patience.
The original Writing Tip 3 on the main site continues into the trickier cases this post didn’t have room for — repeating “the” correctly across a list, and getting “both…and” and “not only…but also” to line up — if you want the fuller reference.
If this is the kind of sentence-level snag you’d rather someone else caught before a journal sees it, that’s the level our editing service works at — line by line, alongside grammar, phrasing and structure generally.
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