Maintaining Parallel Structure (or, One of These Things is Not Like the Other)

Lengthy lists or complex series of items may be difficult for a reader to understand unless every item is in the same grammatical form as the other items. If a series contains three items, all three should be clauses, phrases, or words of the same part of speech (e.g., nouns, verbs, adjectives). Such a list is said to maintain parallel structure and will be clearer to a reader than a list without parallel structure. A series without parallel structure may be revised in a number of ways, but there are two simple approaches that work well in most cases: revising all items to be of the same form or dividing the series into two or more separate series. The examples below provide an illustration of the use of these processes to make a long list easier to read.

Original sentence: The efficacy of ATO has been attributed to the induction of apoptosis, inhibiting proliferation and angiogenesis, and promotion of differentiation.

Suggested correction: The efficacy of ATO has been attributed to the induction of apoptosis, inhibition of proliferation and angiogenesis, and promotion of differentiation.

In the original sentence, the three items vary in form; they are all noun phrases, but one is worded differently from the other two. The revision makes all three items the same type of noun phrase (i.e., they start with nouns ending in ‘-tion' instead of gerunds).

Original sentence: This method is labor intensive, time consuming, expensive, and relies upon the submission of fresh material to the diagnostic laboratory.

Suggested correction: This method is labor intensive, time consuming, and expensive, and it relies upon the submission of fresh material to the diagnostic laboratory.

In the original, the first three items are simple adjectives, but the fourth is a phrase beginning with a verb. The revision groups the three adjectives into one series and places the fourth item in its own independent clause. Dividing a series into two or more separate series works well if it would be difficult to give every item the same form or if the items are too dissimilar.

Original sentence: These symptoms are associated with aging, cancer and other diseases.

Suggested correction: These symptoms are associated with aging and with cancer and other diseases.

Often, the items in a series are all the same form (part of speech, phrase, or clause), but the three items do not truly belong together in a list. In the original, the first item listed is not a type of disease, so it should not be grouped with the other two items.

Original sentence: Additional work was needed to find other materials that are readily available, cost effective and that demonstrate comparable catalytic effects for the tri-iodide reduction in DSSCs.

Suggested correction: Additional work was needed to find other materials that are readily available and cost effective and that demonstrate comparable catalytic effects for the tri-iodide reduction in DSSCs.

The original construction places “cost effective” between two subordinate clauses beginning with ‘that,' making it difficult to determine where the second item belongs. The revision joins the second item with the first.

We hope that this article gives you some ideas for breaking up long lists or ensuring that they maintain proper parallel structure. AJE wishes you the best of luck!