GMAT语法中有一个重要的考点，叫做平行。那么我们今天就来谈一谈平行。
在GMAT语法中，在某些情况下，也希望句子的某些成分能实现平行，达成形式上的对称和一致。比如在出现以下两类词的时候，需要考虑平行：一、类词，我们把它们称为 closed markers (封闭性标志词)，如 not only…but also…from…to...both…and…等；第二类词，我们称之为 open markers (开放性标志词)，如 andbutor 之类。这两类词出现时，首先要保证相应的平行成分，在形式上保持一致。
具体来说，比如在以下这个句子中，形式平行的原则贯彻的十分OK: I not only know him but also know well about him. “not only”和“but also”中间的成分为“know him”，是一个动词短语，but also后面的成分是“know well about him”，也是一个动词短语，形式上非常一致。而以下这个句子就不满足平行的原则：I not only know him but also I know well about him. 在这个句子中，“not only”和“but also”框出的平行成分是“know him”，一个介词短语，但是 but also 后面的是“I know well about him”，一个句子，所以两者形式上不平行。同理，在以下这两句中，句1：“She is not only beautiful but also kind.” 句2：“She is not only beautiful but also is kind.”前一句“beautiful”和“kind”实现了平行，而后一句“beautiful”和“is kind”则不平行。所以总结来说，在 close markers 中，成对出现的这两个词（如 not only 和 but also）把平行成分固定了下来，第二个词后面的成分要和该平行成分形式上保持一致。形式的描述如下：not only 平行成分1 but also 平行成分2，平行成分1、2形式上保持一致。
接下来我们可以看一道例题来进一步熟悉此点：
At the end of the 1930s, Duke Ellington was looking for a composer to assist him –someone not only who could arrange music for his successful big band, but mirroring his eccentric writingstyle as well in order to finishthe many pieces he had started but nevercompleted.
(A) someone not only who could arrange music for his successful big band, but mirroring his eccentric writing style as well in order to finish
(B)someone who could not only arrange music for his successful big band, but also mirrorhis eccentric writing style in order to finish
(C)someone who not only could arrange music for his successful big band, but also to mirror his eccentric writing style in finishing
(D) that being someone who could not only arrange music for his successful big band, but mirroring his eccentric writing style for finishing
(E)beingsomeone not only who could arrange music for his successful big band, but mirroringhis eccentric writing style as well, finishing
大家可以先试着自己运用一下上面的平行原则，去解这道题。

接下来我们分析一下这道题：
At the end of the 1930s, Duke Ellington was looking for a composer to assist him –someonenot onlywho could arrange music for his successful big band,butmirroring his eccentric writing style as well in order to finishthe many pieces he had started but never completed.
(A) someone not onlywho could arrange music forhis successful big band, but mirroring his eccentric writing styleas well in order to finish
(B) someone who could not onlyarrangemusic for his successful big band, but alsomirrorhiseccentric writing stylein order to finish
(C) someone who not onlycould arrange music for his successful big band,but alsoto mirrorhis eccentric writing stylein finishing
(D)that being someone who could not onlyarrange music for his successfulbig band, butmirroringhis eccentricwriting style for finishing
(E) being someone not onlywho could arrange music forhis successful big band, butmirroring hiseccentric writing style as well, finishing
在此题中，首先找到平行的标志词，为“not only…but (also)”, 那么根据以上原则，划出平行成分1、2，末了这两个成分实现了形式上的一致的只有 B选项，“arrange”对“mirror”，动词短语对动词短语，所以正确答案就是B。