Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Balance parallel ideas presented as pairs
Balance parallel ideas
Clarity

When pairing ideas, underscore their connection by expressing them in similar grammatical form. Paired ideas are usually connected with coordinating conjunctions, with correlative conjunctions, or with than or as.

Parallel ideas linked with coordinating conjunctions

Coordinating conjunctions (and, but, or, nor, for, so, and yet) link ideas of equal importance. When those ideas are closely parallel in content, they should be expressed in parallel grammatical form.

Parallel ideas linked with correlative conjunctions

Correlative conjunctions come in pairs: either … or, neither … nor, not only … but also, both … and, whether … or. Make sure that the grammatical structure following the second half of the pair is the same as that following the first half.

Comparisons linked with than or as

In comparisons linked with than or as, the elements being compared should be expressed in parallel grammatical structure.

Comparisons should also be logical and complete. (See 10c.)