Parallelism in Writing

What is parallel sentence structure?

A parallel sentence structure refers to words, phrases, or clauses in the same sentence having the same grammatical form. Parallelism encourages natural flow and is most often applied to sentences that are either listing elements or comparing elements. Parallel sentence structure should also be applied to headings and section titles in academic papers.

Incorrect
I enjoy books, writing, and painting.
correct
I enjoy reading, writing, and painting.

Both of the above sentences list elements. The elements listed by the first sentence are “books,” “writing,” and “painting.” While “writing” and “painting” have the same grammatical form (gerunds), “books” does not (common nouns). As such, this sentence comes off as awkward and somewhat unbalanced. The elements listed by the second sentence are “reading,” “writing,” and “painting.” These elements all share the same grammatical form, and therefore the sentence reads naturally and gives the reader a sense of rhythm.

Incorrect
You can apply either by visiting the office or email.
correct
You can apply either by visiting the office or sending us an email.

These two sentences are comparing elements. The elements compared by the first sentence are “visiting the office” and “email.” These two elements have different grammatical forms (verb phrase and noun). On the other hand, the elements compared by the second sentence – “visiting the office” and “sending us an email” – share the same form.

Parallel Structure in a Series

As mentioned above, elements in a list (or a series) should share the same grammatical form. These elements may be words, phrases, or clauses and are typically connected with the use of commas or semicolons in combination with conjunctions, such as “and” or “or.”

Parallel series of words

When the listed elements are words, the first step to enforcing parallel structure is ensuring that the elements are of the same word type. In other words, nouns should be listed with other nouns, adjectives with other adjectives, and verbs with other verbs.

Incorrect
She bought apples, potatoes, and is going to buy oranges.
correct
She bought apples, potatoes, and oranges.
Incorrect
We are looking for an editor who is organized, self-motivated, and has a good eye for detail.
correct
We are looking for an editor who is organized, self-motivated, and detail-oriented.

If the listed items are verbs, they must all be of the same form. For example, a verb in the infinitive form must be listed with other verbs in the infinitive; it should not be listed with participles or gerunds.

Incorrect
I need to cook, showering, and study.
correct
I need to cook, shower, and study.

Parallel series of phrases

When verbs are listed in series, they are typically in the form of verb phrases. As with elements that are single verbs, elements that are verb phrases must also be of the same grammatical form. In other words, infinitive phrases should be listed with other infinitive phrases, participle phrases should be listed with other participle phrases, gerund phrases should be listed with other gerund phrases, and so on.

Incorrect
Your primary roles are greeting clients, to answer the phone, and updating the company calendar.

“Greeting clients” and “updating the company calendar” are gerund phrases. However, “to answer the phone” is an infinitive phrase. By turning the infinitive phrase “to answer the phone” into the gerund phrase “answering phone calls,” parallel structure is maintained.

correct
Your primary roles are greeting clients, answering phone calls, and updating the company calendar.

Parallel series of clauses

The same principle applies to elements that are verb clauses; all elements must be of the same grammatical form.

Incorrect
The children were told that they should not run in the halls, that they should not chew with their mouths open, and to not use profanities.

“That they should not run in the halls” and “that they should not chew with their mouths open” share the same grammatical form. However, “to use profanities” does not. To enforce parallel structure, it is necessary to alter “to use profanities” so that it has the same grammatical form as the earlier clauses. Or, alternatively, the other two clauses could be altered to have the same form as “to use profanities.”

correct
The children were told that they should not run in the halls, that they should not chew with their mouths open, and that they should not use profanities.
correct
The children were told to not run in the halls, to not chew with their mouths open, and to not use profanities.

Parallel lists after colons

Lists that come after colons must also have parallel elements.

Incorrect
The fabricated composites were found to share three desirable characteristics: having a high resistance to heat, low brittleness, and high flexibility.
correct
The fabricated composites were found to share three desirable characteristics: high heat resistance, low brittleness, and high flexibility.

Parallelism in Pairs

As is the case for elements in a list, elements in pairs must also be parallel. Elements in pairs may be connected by coordinating conjunctions, correlative conjunctions, or comparative words. There are seven coordinating conjunctions: and, or, nor, but, yet, so, for. All of these can be used to connect elements into pairs. As is the case for lists, both elements in the pair must share the same grammatical form.

Incorrect
We went shopping and to bike.
correct
We went shopping and biking.

Correlative conjunctions are pairs of words that connect elements in a sentence. There are five correlative conjunctions: either…or, neither…nor, whether… or, not only… but also, both…and.

Incorrect
The samples can be collected with either a cotton swab or puncturing the epidermis with a needle.
correct
The samples can be collected with either a cotton swab or a needle.
correct
The samples can be collected by either swiping the inner cheek with a cotton swab or puncturing the epidermis with a needle.

Two elements that are being compared in a sentence must also be parallel. Elements are compared using words such as “than,” “as,” “like,” and “over.”

Incorrect
We decided to examine the samples individually rather than scanning them all at once.
correct
We decided to examine the samples individually rather than to scan them all at once.

Parallelism in Headings and Outline Sections

The section headings of a given academic paper should have the same grammatical structure. Although journals do not typically state this as part of their author guidelines, enforcing parallel structure in section headings helps a paper come off as more professional and organized. The same applies to a paper’s outlines.

Incorrect
Symptoms and disease duration
Mortality rate is high worldwide
Transmission mechanism and the effect of immunizations
correct
Symptoms and disease duration
Mortality rates worldwide
Transmission mechanism and the effect of immunizations

Parallel Subject Matter

A sentence can have elements with the same grammatical form yet still be non-parallel if the elements are not consistent or compatible. All elements in a sentence should be clearly related to one another.

Incorrect
We found that the experimental results agreed with the theoretical model and went out dancing in the city.

Grammatically, “found that the experimental results agreed with the theoretical model” and “went out dancing in the city” are of the same form. However, it is unclear how these two elements are related to one another. For proper parallelism, the connection between the two elements must be clear.

correct
We found that the experimental results agreed with the theoretical model, and to celebrate, we went out dancing in the city.