Words or phrases parallel in meaning should be parallel in form. There are few easier ways to improve the logic, clarity, and style of a sentence. We can see how this rule works if we clumsily alter the first sentence of this paragraph to read: "Words and phrases parallel in meaning should be parallel as far as form goes." This second version makes prompt understanding difficult, because it removes the similarity in form of "in meaning" and "in form." Readers of the first version can easily connect two phrases so similar in appearance.
Comparisons work in the same way. Terms to be compared must be formally (and logically) identical: write "Odysseus's fate is less harsh than that of Aeneas," not "Odysseus's fate is less harsh than Aeneas." "Odysseus's fate is less harsh than Aeneas's" is still another possibility.
Errors in parallelism often arise when students use pairs of conjunctions (also called "correlatives"): "both...and," "either...or," "neither...nor," "not only...but also," and so forth. Such conjunctions should always join identical parts of speech, whether the paired parts of speech are single words, phrases, or clauses.
See Purdue handout
Copyright 1996 Yale University. Revised on Monday, May 20, 1996