明雜劇北曲的聯套規律，與元人之用法不同，已是自明代學者即己明確指出之事實，但此中變異之具體描述，與原理之析論仍有待補充。本文先就觀念進行釐清，並說明討論北曲聯套規律的變化，可有三個不同層次：全劇之宮調搭配、單折（齣）之套式運用、聯套單位之組成與用法。其次具體列舉六項在明雜劇中北曲聯套與元代用法不同的問題：宮調與折次的搭配、【越調】的使用比例、【黃鐘】的用法、【般涉調】的用法、套式的簡化趨同、〔寄生草〕曲牌的用法，具體列舉劇例，分析其與元代差異之處，及其可能的原因。經本文分析可得知，元明兩代之變異，其中有明人不明北曲之理，誤用之處，但亦有雖為創新，但暗合北曲原理的用法；有些則是南北曲交化的影響；有的則為基於戲劇原理、演出需求的創造。

The differences, or variations, of the composing rules of the Northern-Tune songsets, between the Northern Tzarjiuhs in Yuan and Ming dynasties, had been found early in Ming dynasty. But from Ming dynasty till now, a concretely and precisely detailed description about the variations is still in absence.This essay specifies six issues about the variations: 1, The relationship between kung-tiao (tonality) and act order; 2, the percentage of the plays that includes a Yueh-Tiao act; 3, the application of Huang-Chung-Kung; 4, the application of Pan-She-Tiao; 5, the tendency towards simplification and assimilation of songset patterns; 6, the application of Chi-Sheng-Tsao. For each issue, this essay tries to make a concretely and precisely detailed description.In further analysis of these six issues, the predictions about the reasons of these variations are made. And this essay tries to present a new concept about the variations of the composing rules of the Northern-Tune songsets, between the Northern Tzarjiuhs in Yuan and Ming dynasties.