Such an arrangement [syntactic] that one element of equal importance with another is similarly developed and phrased. The principle of parallelism dictates that coordinate ideas should have coordinate presentation. Within a sentence, for instance, where several elements of equal importance are to be expressed, if one element is cast in a relative clause the others should be expressed in relative clauses. Conversely, of course, the principle of parallelism demands that unequal elements should not be expressed in similar constructions. Practiced writers are not likely to attempt, for example, the comparison of positive and negative statements, of inverted and uninverted constructions of dependent and independent clauses. And, for an example of simple parallelism, the sentence immediately preceding may serve. Some departures from parallelism may qualify as “slight of ‘and,'” but most are just signs of carelessness, as when someone says, “I don’t like to fish or swimming.” But a deliberate violation of parallelism can be highly dramatic. Consider the couplet in Houseman’s “Hell Gate” in which coordinating conjunctions (polysyndeton) create the expectation of a parallel group of active verbs but abruptly end with a linked verb: “Then the sentry turned his head,/Looked, and knew me, and was Ned.” (Handbook to Literature)
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