Parallelism is critical to good writing: It allows the flow of the work to proceed unimpeded by reader confusion, it promotes logical thinking on your part as you write and as the reader reads, and it helps you remember to include all the aspects needed to make your point. Parallelism can be within a sentence, within a topic’s discussion, within a list, or throughout the whole work. Let’s look at that sentence, for instance. I might have written: Parallelism can be within a sentence, discussion, list, or work. That sentence is parallel because the preposition and article (within and a) can be placed comfortably before each of the nouns in the series. In the original sentence, I chose to repeat the prepositions because I felt that the last element in the original list (the whole work) needed a slightly different preposition for the greatest clarity. That’s the rule: Either the preposition is for all the elements in the list, or you provide a preposition for each element. I also felt that the sentence was clearer when each element got an article (a or the) and an occasional adjective. With the repeated prepositions, I was able to provide greater description for some of the elements without spoiling the parallelism. Parallelism works both for what you leave out and for what you put in. Parallelism within a topic’s discussion means that you follow the same organizing principle for each section and that each of the sub-topics are relevant to the main topic in much the same way. Imagine that you’re writing a book about African Dwarf Frogs. Parallel discussions of care and feeding, longevity, and breeding are essential. Maybe a short history or genealogical discussion might come into it. But a chapter on how African Dwarf Clawed Frogs (an unrelated species) are taking over a pond in San Francisco’s Golden Gate Park is completely out of place and unrelated. If you’re making a case for an efficient way of programming, an exploration of each subject that’s identical in structure to its opposite number is much more compelling than if one topic is covered chronologically and the other is covered functionally. The reader’s mind makes logical leaps that way, without as much prodding on your part. Parallelism within a list is probably the easiest to maintain. Introduce the list in such a way that each item in the list can have the same sentence (or phrase) structure. This is wrong: Making music is: A nice way to spend time with friends Satisfy your creative urges Building a puzzle is easier. There’s nothing in that list that isn’t true, exactly; the problem is that only one item (the first) goes grammatically with the lead-in. Made parallel, the list could be: Making music is: A nice way to spend time with friends Satisfying creatively Harder than building a puzzle. This would be a stronger list if the structure within each item were also parallel, perhaps starting each one with a verb. Try this one, noticing that even the form of verbs chosen is parallel: Making music is: Socializing with friends Satisfying creatively Challenging Of course, you don’t have to start with a verb each time. You could make each item a complete sentence that uses the same lead in grammatically, you could start each item with an adverb or adjective, or you could make your decision based on the phrasing of the most important item, and perhaps change the lead-in accordingly. Parallelism throughout the whole work is essential. Without it and your organizing principle, your efforts are just a collection of related facts, like a laundry list.