As per the principle of parallelism/parallel structures, comparable parts of the sentence must be structurally and logically similar. To construct a sentence that is parallel in structure, all phrase, verbs, clauses and other elements must be parallel to each other.

Here’s an example:

e.g. Ajinkya likes to run, to swim and to ride a bicycle.

Note that here, all the words – run, swim and ride – are in the infinitive form. Hence, this sentence follows the principle of parallelism.

But consider this sentence:

e.g. Ajinkya likes running, to swim and to ride a bicycle.

Here running is in the –ing form (gerund) whereas the other words ‘swim’ and ‘ride’ follow the infinitive form. Hence, this sentence is not following the parallel structure and is grammatically incorrect.

Markers of Parallelism

Usually, parallel elements will be joined by coordinating conjunctions. The most important coordinating conjunctions are ‘and’, ‘or’ and ‘but’. By spotting these coordinating conjunctions, the parallel elements can be determined in many questions.

Example (Source: Official Guide for GMAT)

Directions: Select the answer choice that produces the most effective sentence when it replaces the underlined part.

1. Some bat caves, like honeybee hives, have residents that take on different duties such as defending the entrance, acting as sentinels and to sound a warning at the approach of danger, and scouting outside the cave for new food and roosting sites.

(A) acting as sentinels and to sound

(B) acting as sentinels and sounding

(C) to act as sentinels and sound

(D) to act as sentinels and to sound

(E) to act as a sentinel sounding

Answer: As we have already said, the elements ‘acting as sentinels’ and ‘to sound a warning’ have to be parallel. Hence, the verb ‘sound’ should be in the –ing form as the other verb ‘act’. Hence, the correct element will be ‘sounding a warning’ and hence the right answer option is B.

Other Examples:

Here are a few more examples. Try them out and post your answer in the comments below. I would recommend solving questions from the GMAT Official Guide to improve your sentence correction abilities, not only for parallelism but for other topics as well.

1. Like ants, termites have an elaborate social structure in which a few individuals reproduce and the rest are serving the colony by tending juveniles, gathering food, building the nest, or they battle intruders.

A. are serving the colony by tending juveniles, gathering food, building the nest, or they battle

B. are serving the colony in that they tend juveniles, gather food, build the nest, or battle

C. serve the colony, tending juveniles, gathering food, building the nest, or by battling

D. serve the colony by tending juveniles, gathering food, by building the nest, or by battling

E. serve the colony by tending juveniles, gathering food, building the nest, or battling

2. It is well known in the supermarket industry that how items are placed on shelves and the frequency of inventory turnovers can be crucial to profits.

A. the frequency of inventory turnovers can be

B. the frequency of inventory turnovers is often

C. the frequency with which the inventory turns over is often

D. how frequently is the inventory turned over are often

E. how frequently the inventory turns over can be

3. Eating saltwater fish may significantly reduce the risk of heart attacks and also aid for sufferers of rheumatoid arthritis and asthma, according to three research studies published in the New England Journal of Medicine.

A. significantly reduce the risk of heart attacks and also aid for

B. be significant in reducing the risk of heart attacks and aid for

C. significantly reduce the risk of heart attacks and aid

D. cause a significant reduction in the risk of heart attacks and aid to

E. significantly reduce the risk of heart attacks as well as aiding