1. SYNTAX OF THE ENGLISH LANGUAGE
PARALLELISM

2. PARALLELISM
Parallelism is an important syntactic consideration. To quote Tichy and Fourdrinier (1988),
A major device for sentence emphasis is parallel construction. Equal thoughts demand expression in the same grammatical form. Repetition of structure within a sentence is a most effective device for making the long sentence easy to read, and repetition of structure in two or more sentences connects them. An understanding of parallelism is therefore essential for emphasis and coherence.

3. PARALLELISM
When should sentence elements be parallel and how do we make them so? When two or more ideas are logically equal, they should be made parallel by writing them in the same grammatical structure. Grammatically, words are equal (parallel) to words, phrases to phrases, subordinate clauses to subordinate clauses, and independent clauses to independent clauses.
For example, the two coordinate clauses in the following sentence are not logically equal:

4. PARALLELISM
The compressor may be operated in the compression mode and then the flow is expelled from the anechoic room to the test duct.
This sentence calls for subordination, not coordination:
When the compressor operates in the compression mode, the flow is expelled from the anechoic room to the test duct.

5. CONNECTIVES REQUIRING PARALLELISM
Linton (1962) lists four types of connectives requiring parallelism: coordinate conjunctions (and, or, but), correlative conjunctions (either or, both and, not only but also), conjunctive adverbs (therefore, otherwise, however), and the semicolon used to connect independent clauses.

6. CONNECTIVES REQUIRING PARALLELISM
Coordinate conjunctions probably provide the most opportunities to use parallelism; they join words, phrases, and clauses of equal grammatical rank. Coordinate clauses joined by a coordinate conjunction should be logically equal. Similarity in grammatical structure, if possible, is also a good idea. For example, the voice of the verb might be kept the same:

7. CONNECTIVES REQUIRING PARALLELISM
Acceptable The mixing noise is the dominant component of the spectrum, but the background noise peaks at a high frequency.
Better The mixing noise dominates the spectrum, but the background noise peaks at a high frequency.

8. CONNECTIVES REQUIRING PARALLELISM
Correlative conjunctions demand strict parallelism: Both elements of the correlative must be followed by the same part of speech. See the examples:
Both my husband and my father are very fond of soccer.
Not only did he fail but he also had to drop out of school.