平行結構

這個平行結構的原則要求內容和功能相似的詞句在表面上和外觀上必須相像.。此一詞句型態的相像將使讀者更易於辨認內容和功能的相似性。

寫作技巧不純熟的人經常違反此原則，因為他們誤以為他們應經常改變詞句的型態。誠然，在重複一項敘述時，為了強調它，寫作者或許必須改變它的型態。但除此之外，寫作者應恪遵這個平行結構原則。

上聯不對下聯	上下聯相對
Formerly, science was taught by the textbook method, while now the laboratory method is employed.	Formerly, science was taught by the textbook method; now it is taught by the laboratory method.

左邊的句子給人寫作者優柔寡斷或膽怯的印象；他似乎不能或不敢選擇一種詞句的型態並堅守之。右邊的句子顯示寫作者至少做了這項選擇並堅守之。

根據這項原則，在一系列詞句中，冠詞或介系詞只能放在第一個詞的前面，或者每個詞的前面都加冠詞或介系詞。

上聯不對下聯	上下聯相對
The French, the Italians, Spanish, and Portuguese	The French, the Italians, the Spanish, and the Portuguese
In spring, summer, or in winter	In spring, summer, or winter (In spring, in summer, or in winter)

相關連接詞 (Correlative Conjunctions：both, and; not, but; not only, but also; either, or; first, second, third; and the like) 後面必須接相同的文法結構。許多違反這個規則的句子可以藉著重新排列句子來加以修正。

上聯不對下聯	上下聯相對
It was both a long ceremony and very tedious.	The ceremony was both long and tedious.
A time not for words, but action	A time not for words, but for action
Either you must grant his request or incur his ill will.	You must either grant his request or incur his ill will.
My objections are, first, the injustice of the measure; second, that it is unconstitutional.	My objections are, first, that the measure is unjust; second, that it is unconstitutional.

上聯不對下聯	上下聯相對
My income is smaller than my wife.	My income is smaller than my wife's.