Parallelism occurs when words that serve the same function in a sentence take the same form. Readers find parallel constructions easier to understand than those that are not parallel.
Here are some examples of word forms that commonly appear in parallel structures:
- Gerunds: traveling, eating, walking
- Infinitives: to write, to ponder, to revise
- Adverbs: quickly, slowly, craftily
Now compare a sentence that uses parallel structure to one that doesn't:
- Parallel: Important skills to cultivate in graduate school include thinking, writing, and revising.
- Not parallel: Important skills to cultivate in graduate school include thinking, writing, and to revise.
The second one feels a little bit like stumbling on an uneven sidewalk, while the first follows through on the pattern it creates, allowing us to coast smoothly all the way to the end.