Parts of sentences are said to be parallel when they are connected by commas (in a list) or by conjunctions such as and, or, or but and have the same grammatical form (see Parallelism for a simple explanation). Parallelism repeats lexis or structure to make it easier for readers to understand new information because it is given in a familiar form. The first segment establishes a structure that is then repeated so that ideas are expressed through lexical and syntactic parallels. Carried to an extreme, it is a powerful device that is often used for rhetorical effect. Many famous quotes are remembered not only because they were uttered by important people at important times but because they have a distinct parallel structure.



In the first example, Winston Churchill coordinates a series of four one-syllable nouns as the objects of the preposition to give an aesthetically pleasing and powerful message. The force of his argument would have been lost if he had used, for example, just the noun phrase hard work. In the second, he coordinates whole verb clauses, the subject and verb of which are repeated and combined with different prepositional phrases, to generate a clear rhythm and a strong emotional effect.

In the quote below, Abraham Lincoln coordinates three prepositional phrases, in which the preposition varies but the object remains the same, to highlight the nature of democracy.


But parallel structure is also a useful communicative tool for researchers because it clarifies meaning and makes ideas more comprehensible and memorable for readers. It is particularly important in lists and coordinated sentences. For example, in the sentence below, the subordinating conjunction because introduces a non-parallel list.


The first and second items are verb clauses (subject + verb + complement), but the third item (high labour costs) is a noun phrase. Readers will probably have to read the sentence two or three times before they realize that it has no accompanying verb and that the list has three items, one of which is not parallel to the others. Changing the third item to a verb clause solves the problem.  


Likewise, in the sentence below, two very similar ideas are expressed with two different grammatical structures (infinitive phrase and verb clause).


The message can be more efficiently expressed if the grammatical form of the second idea mirrors that of the first.


Parallel structure can be particularly valuable in results and discussion sections where data, ideas and situations are contrasted, and differences and similarities are highlighted. Parallel structure makes these differences and similarities easier to see because the constant remains constant and only the variable varies. That is to say, when the words are different, the ideas are different; when the words are the same, the ideas are the same. By presenting your data in parallel sentences, you can point readers directly to your findings.



Finally, a word of warning. As mentioned above, in parallel structures be sure that every coordinated item has the same grammatical form.



Ensure, however, that readers can clearly discern where parallel segments begin and end. In the following sentence, it is not clear where the parallelism begins.


It seems to be saying that Injury to endothelial cells increases sensitivity to three coordinated segments. This is not the case. It increases three coordinated segments, one of which is sensitivity to vasopressor agents. The problem can be solved by repositioning the problematic segment.