Parallel structures are sequences of coordinated words, phrases, or clauses that are of the same grammatical structure. Parallel structure can be applied to a single sentence, a paragraph, or even multiple paragraphs. Compare the two following sentences:

• Rural landlords wanted income from rent, to farm parts of their own estate, and being middlemen for agricultural trade.
• Rural landlords wanted income from rent, from farming parts of their own estate, and from being middlemen for agricultural trade.

Most people find the second sentence easier to comprehend than the first because the second sentence uses parallelism—the “from” launches three different forms of desired income, so that the reader easily understands that rent, farming, and being middlemen are all options. The first sentence is more confusing because the concepts don’t line up grammatically: the first and third options (rent and being middlemen) have “wanted income from” as their root; the second time (to farm parts) has perhaps only “wanted” as its root. While the first sentence is technically okay, it’s awkward and makes it easy to trip up over the mismatching items. Parallelism improves writing style and readability, and it makes sentences easier to process.

Compare the following examples:

• Lacking parallelism: “She likes cooking, jogging, and to read.”
  • Parallel: “She likes cooking, jogging, and reading.”
  • Parallel: “She likes to cook, jog, and read.”
• Lacking parallelism: “He likes to swim and running.”
  • Parallel: “He likes to swim and to run.”
  • Parallel: “He likes swimming and running.”

Once again, the examples above combine gerunds (a noun formed by adding -ing to a verb) and infinitives (the form a verb takes when preceded by to). To make them parallel, the sentences should be rewritten with just gerunds or just infinitives. Note that the first nonparallel example, while inelegantly worded, is grammatically correct: “cooking,” “jogging,” and “to read” are all grammatically valid conclusions to “She likes.”

Parallelism can also apply to names. If you’re writing a research paper that includes references to several different authors, be consistent in your references. For example, if you talk about Jane Goodall and Henry Harlow, you should say “Goodall and Harlow,” not “Jane and Harlow” or “Goodall and Henry.” This is something that would carry on through your entire paper: you should use the same mode of address for every person you mention.

You can also apply parallelism across a passage:

Manuel painted eight paintings in the last week. Jennifer sculpted five statues in the last month. Zama wrote fifteen songs in the last two months.

Each of the sentences in the preceding paragraph has the same structure: Name + -ed verb + number of things + in the past time period.

Here’s a more sophisticated example that’s useful, too, because it makes a move that often occurs in academic writing: elaborating on an initial claim. This example is adapted from Keith Wrightson’s English Society 1580-1680 (p. 134):

Those best placed to prosper were the yeomanry, whether they were freeholders owning their own land or substantial tenants. If freeholders, they were immune from the rent increases of the period. If large tenants, they were commonly profiting from their farming enough to afford rent increases with comparative ease.

Parallelism and Repetition

Parallelism can also involve repeated words or repeated phrases. Inexperienced writers often think that repetition should be avoided at all costs, probably because someone once warned them, usefully enough, about redundancy. However, in sophisticated writing, repetition is a way to highlight what is important. You should repeat keywords frequently enough throughout an essay that your reader is in no danger of losing your thread; parallelism offers a further opportunity to create cohesion and to emphasize crucial points. Here are a few rhetorically powerful examples of repetition in parallel positions:

• “The inherent vice of capitalism is the unequal sharing of blessings; the inherent virtue of socialism is the equal sharing of miseries.” —Winston Churchill
• “Let every nation know, whether it wishes us well or ill, that we shall pay any price, bear any burden, meet any hardship, support any friend, oppose any foe to assure the survival and the success of liberty.” —John F. Kennedy
• “And that government of the people, by the people, for the people, shall not perish from the earth.” —Abraham Lincoln, Gettysburg Address

These repeated phrases seem to bind the work together and make it more powerful—and more inspiring. This use of parallelism can be especially useful in writing conclusions of academic papers.