Parallelism is an aspect of coordination. Parallel (from Greek) means “side by side.” The general principle is that parallel elements must have exactly the same structure; otherwise, they won’t match. Take the following examples:
ERROR: The fleas wanted the final number rechoreographed, music recomposed, and audience deloused. [To be parallel, all the items in the series should be preceded by the.]
The fleas wanted the final number rechoreographed, the music recomposed, and the audience deloused.
ERROR: The hero wanted to break away from a dominating father and finding independence. [Two non-finite verb phrases designed to be coordinate completers of wanted. But the infinitive phrase to break away doesn’t match the -ing participle phrase finding.]
The hero wanted to break away from a dominating father and find independence. [Note that now the infinitive marker to serves both break away and find.]
ERROR: Today, there are many articles dealing with cobras, supplying intimate details about their hissing techniques and when they try to give each other hickeys. [Two completers, connected by and, of the preposition about, but the noun phrase hissing techniques isn’t parallel with the “when” clause.]
… supplying intimate details about such things as their hissing techniques and their attempts to give each other hickeys. [Techniques and attempts are the main words of the parallel noun phrases.]
ERROR: Machiavelli’s “The Prince” explains that some men are marked with qualities that either bring them blame or praise: liberal or stingy, openhanded or cruel, compassionate or breaker of faith, coward or vigorous and spirited.
There are a number of faults of parallelism in the preceding sentence, which comes from the first draft of an essay. First, the writer needed to have the phrases after the colon to be in apposition with qualities. But qualities is a noun, and except for coward (which I’ll come back to), the series after the colon is composed of adjectives. Secondly, blame comes before praise, but in the listing of traits, the praiseworthy ones are placed before the blameworthy ones. Next, the paired qualities should be opposites (liberal and open-handed both mean “generous” in this context). And finally, although coward is a noun, it’s a noun that refers to a person and not to a quality.
Machiavelli’s “The Prince” explains that some men are marked with qualities that either bring them praise or blame: these are liberality or stinginess, compassion or cruelty, loyalty or treachery, and courage or cowardice.
ERROR: Fang exercises to develop his venom and for the nice tingling in his scales. [The two phrases, which are connected by the coordinating connective and, modify exercises. But the first is a non-finite verb phrase and the second is a prepositional phrase.]
Fang exercises to develop his venom and to enjoy the nice tingling in his scales.
In making comparisons, we need to be careful that the items being compared are parallel:
ERROR: When the Green Knight’s traits are compared to Sir Gawain, a definite mix in his qualities becomes apparent. [We need to compare traits to traits, not traits to a person.]
When Sir Gawain’s traits are compared to those of the Green Knight, inconsistencies in Gawain’s character become apparent.
ERROR: Fang thought that keeping the championship was less fun than to relax in the sauna. [Keeping is an -ing participle; to relax is an infinitive.]
Fang thought that keeping the championship was less fun than relaxing in the sauna.
With correlatives (italicized in the following examples), the trick is to get the correlating adverb correctly placed:
ERROR: He not only kisses princesses but also salamanders.
He kisses not only princesses but also salamanders. [Now “not only…but also” correctly indicates coordinate completers.]
He not only kisses princesses but also tickles piranhas. [Now the correlative correctly indicates coordinate finite verbs.]
ERROR: We’d love to hold both a frog-kissing contest and to broadcast it live. [We have to move both so that it modifies both infinitives.]
We’d love both to hold a frog-kissing contest and to broadcast it live.
We’d love to hold both a frog-kissing contest and a salamander-licking derby.