The primary purpose of writing parallel ideas in parallel structure
is to maximize the use of absorption on the part of the reader.
Parallel structure sets up a rhythm that allows the reader to easily absorb
a longer and complex series of information.
In the English linguistic community, all good writers
use parallel structures and all readers expect these structures.
Whenever two or more items are in parallel position, with the horizontal or
within a sentence in a paragraph or vertical in a bullet list,
the principle of parallel structure applies.
Here are some examples of good parallel structure.
While each of the sentences present the same information slightly differently,
all the sentences represent correct parallel structures.
By contrast, consider the following sentence.
I was working as an engineer and trained as a biologist.
The sentence both looks wrong and sounds wrong.
The problem is that while was working and
trained are both predicates, they are not in the same grammatical form.
Was riding, his bus continues, and trained is past simple.
Thus the sentence lacks parallel structure.
Both its grammar and its style are unacceptable.
Instead, the writer should use either I was working as an engineer and
training as a biologist, or I worked as an engineer and trained as a biologist.
One aspect of parallelism has to do with meaning.
Having three parallel words whether in a sentence or
in a vertical list that are all nouns or all in verbs or
all preposition phrases is correct only if all three belong to the same list.
Grammatical parallelism is not enough.
Let's consider for example, the series metal, alloy and titanium.
While all three are nouns, the first two nouns are generic nouns,
whereas the third is a specific noun within one of those generic categories.
Therefore, the list is not properly parallel.
The writer must consider whether to write, metals, alloys and
metal compounds or copper, palladium or titanium.
Sometimes, which words or
concepts are parallel with others escapes the writer's initial notice.
In lists the missing word or phrase often appears in a sentence beginning with also.
Good academic writers never begin an English sentence with also, but
rather with in addition, additionally, moreover or furthermore.
A sentence begins with also is almost always a sign of the first draft,
indicating that information following also belongs in the previous sentence.