Writing can be a lot more like math than first meets the eye, especially in regard to sentence structure.
Think of coordinating conjunctions (and, but, or), for instance, like plus signs. If you studied algebra, you’ll recall that different variables can’t be added. So an equation like 2X + 5Y= 60 can’t be restated as 7Y=60 or 7X=60. In other words, you can’t add apples and oranges.
So too in writing, apples and oranges can’t be “added,” that is, they can’t be on two sides of a coordinating conjunction.
That’s the principle. Now how does this concept play out in real writing?
Here are a few ways the principle of parallelism should work:
1. In a list, all items should be the same part of speech.
Poor: Her favorite parts were the actors, the scenery, and how funny the movie was. (joins two nouns and one clause)
Better: Her favorite parts were the actors, the scenery, and the humor.
2. When a conjunction is joining two prepositional phrases, the preposition needs to be included.
Poor: A crisis is not a time for wasteful discussion, but meaningful action. (omits a preposition)
Better: A crisis is not a time for wasteful discussion, but for meaningful action.
3. In compound sentences, the various clauses should be either active or passive rather than mixed.
Poor: Authors may leave a message in the comment box, or emailing a request is accepted. (first clause is active, second is passive)
Better: Authors may leave a message in the comment box or email a request. (compound sentence changed to a compound verb)
Proper parallel construction, which may include intentional repetition, can be a valuable tool in writing. It paces the writing, shows contrasts or comparisons, provides emphasis, and more.
The Bible, especially the King James Version, is a great source for examples of parallelism. Here’s one example from the book of Proverbs in the New American Standard. Note how the two clauses start with prepositional phrases using the same prepositions:
At the head of the noisy streets she cries out;
At the entrance of the gates in the city she utters her sayings
My advice? Read Scripture regularly. It’s a great spiritual discipline, but it also provides wonderful examples of parallelism, to the point that your use of it will become instinctive.