When elements are parallel in meaning and in grammatical function, they also should be parallel in grammatical form. For example, in She liked reading and writing, the words reading and writing have similar roles because they are both objects of the verb liked , and they are grammatically similar because they are both gerunds.
Swimming is better exercise than to ski.
Swimming is better exercise than skiing. They came on foot, by car, and by bicycle. Use parallel words, phrases, clauses, verbs, and tenses to improve the flow of ideas and heighten effect. Similarity of form helps readers recognize similarity of content or function. Use parallel construction in lists, bulleted items, or headings.