Parallel structure(also called parallelism) is the repetition of a chosen grammatical form within a sentence. By making each compared item or idea in your sentence follow the same grammatical pattern, you create a parallel construction.
USING PARALLEL STRUCTOR AITH CORRELATIVE CONJUNCTIONS
When you correct two clauses or phrases with a correlative conjunctions (not only...but also, either... Or, neither... Nor, if... Then), use parallel structure.