北曲與南曲用韻最大差異在於入聲。北曲入派三聲，南曲入聲獨立。入派三聲與入聲獨立，分別運用於南北曲創作，主要體現於協韻律和平仄律。以北曲為例，北曲譜律若當用平聲而取入聲作平聲之字，謂之「以入作平」，朱權《太和正音譜》標示「作平」。南曲譜律若當用平聲而用字窮乏時，可用入聲字替代，謂之「入代平聲」或「以入代平」，沈璟《增定南九宮曲譜》標示「作平」。故南北曲譜所謂「作平」，涵義不同。王驥德《曲律‧論平仄》認為沈璟僅止於「以入代平」，指出譜律若當用平上去三聲而用字窮乏時，可用入聲字替代，本文以「入代三聲」一詞概括，視為王驥德提出的新概念，在南曲入聲創作論發展史上具有承上啟下的樞紐地位。「入派三聲」、「入代平聲」、「入代三聲」運用於填曲度曲，皆需與南北曲文字譜對照方能彰顯。這三者原屬創作論，當其播之於歌樂時，如何體現北曲「入派三聲」，又如何體現南曲「入代平聲」和「入代三聲」，成為度曲論的範疇。本文融貫元、明、清相關論述，深入爬梳；並結合創作論與度曲論，運用戲曲音韻釐清相關概念。從「入派三聲」說到「入代平聲」、「入代三聲」，用以論述入聲字在填詞度曲中的原則與通變，彰顯曲韻曲學的歷史觀照與學術意義

The differences between the rhymes used in northern and southern traditional drama hinge upon entering tones. In northern traditional drama, entering tones are recategorized into level, rising, and departing tones, whereas they are independent in southern traditional drama. These differences become manifest in the accommodation of rhymes and tonal patterns. In northern traditional drama, for example, it is called yi ru zuoping when characters with entering tones are used as level tones. Zhu Quan (1378-1448) marks this as zuoping in his Taihe zhengyin pu (The Formulary of Correct Sounds for Great Harmony). However, in southern traditional drama, characters with entering tones are used to replace those with level tones, a practice called ru dai pingsheng or yi ru daiping. In Shen Jing's (1553-1610) Zengding nan-jiu-gong qüpu (Supplement to the Southern Nine First Notes Music Book), the replacement of tones in southern traditional drama is also called zuoping, although it is completely different from that used in the northern tradition. Wang Jide's (1560?-1623) ＂Lun pingze＂ (Discussion of Tonal Patterns) in the Qülü points out that Shen Jing was referring to yi ru daiping, meaning characters with entering tones that replaced level, rising, and departing tones when no characters with these three tones were suitable. Hence, in this article, ru dai sansheng is used to refer to the new conception advanced by Wang, as this idea plays a key role in the development of entering-tone composition in southern traditional drama. Ru pai sansheng, ru dai pingsheng, and ru dai sansheng in qü composition are only clearly revealed when the music books of southern and northern traditional drama are compared. They were originally regarded as theories of composition, and only later became incorporated into northern and southern duqü theory. This article discusses related discourses about drama from the Yuan, Ming and Qing dynasties that concern theories of composition and duqü in an effort to clarify several critical concepts. It investigates the principles informing entering tones in qü composition through an analysis of ru pai sansheng, ru dai pingsheng and ru dai sansheng. In so doing, this article aims to highlight the academic importance of qü studies from a historical perspective