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Parallelism (grammar) In grammar, parallelism, also known as parallel structure or parallel construction, is a balance within one or more sentences of similar phrases or clauses that have the same grammatical structure. The application of parallelism improves writing style and readability, and is thought to make sentences easier to process.
The definition of parallel is that the sum of the angles on each side of the transversal of two lines is two right angles. From this we can show that the alternate interior angles are congruent. In our case, we have that the alternate interior angles are congruent, that is, we have that there are two pairs of alternate interior angles that are congruent, and that together one angle from each pair forms a straight angle, as you go along the lines.