What exactly is parallel structure? It’s simply the practice of using the same structures or forms multiple times: making sure the parts are parallel to each other. Parallel structure can be applied to a single sentence, a paragraph, or even multiple paragraphs. Compare the two following sentences:

• Yara loves running, to swim, and biking.
• Yara loves running, swimming, and biking.

Was the second sentence easier to comprehend than the first? The second sentence uses parallelism—all three verbs are gerunds, whereas in the first sentence two are gerunds and one is an infinitive. While the first sentence is technically correct, it’s easy to trip up over the mismatching items. The application of 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 and infinitives. 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.”

• Lacking parallelism: “The dog ran across the yard, jumped over the fence, and down the alley sprinted.”
• Grammatical but not employing parallelism: “The dog ran across the yard and jumped over the fence, and down the alley he sprinted.”
• Parallel: “The dog ran across the yard, jumped over the fence, and sprinted down the alley.”

The nonparallel example above is not grammatically correct: “down the alley sprinted” is not a grammatically valid conclusion to “The dog.” The second example, which does not attempt to employ parallelism in its conclusion, is grammatically valid; “down the alley he sprinted” is an entirely separate clause.

Parallelism can also apply to names. If you’re writing a research paper that includes references to several different authors, you should 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.”

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. When using parallelism across multiple sentences, be sure that you’re using it well. If you aren’t careful, you can stray into being repetitive. Unfortunately, really the only way to test this is by re-reading the passage and seeing if it “feels right.” While this test doesn’t have any rules to it, it can often help.