明代萬曆以後，劇壇盛行崑山腔，晚明江蘇松陵沈寵綏（？-1645）編撰《度曲須知》（1639）。崑山水磨調講究「啟口輕圓，收音純細」，沈寵綏乃以中原之音為「體」，吳語為「用」，創立「三字切法」，又稱「三音合切」。這是用三個字拼切出一個漢字的讀音，上一字即字頭，中一字即字腹，下一字即字尾。沈寵綏建構崑腔「切法即唱法」有其理路。首先，將拼切字音的方法分為三類：第一類「頭腹尾三音合切」；第二類「頭腹兩音為切」；第三類「首尾兩音為切、首尾無異音」。其次，強調字頭、字腹、字尾可辨認被切字的語音成分，同韻部之內的切語，雖然字腹、字尾相同，然憑藉字頭可辨認被切字聲母的清濁；而由字腹、字尾可辨認被切字的陰陽調。其三，運用聲母口訣、出字口訣、收音口訣等相關論述，辨析中原音系和吳語音系之差異。這些方法意在教導吳人歌唱崑曲可以精準地掌握字音。三字切法的便捷法門，使歌者既便於掌握字頭（含聲母和介音）、字腹（主要元音）、字尾（韻尾），又不遺漏音節包含的成分，證成沈寵綏建構「切法即唱法」的理論，實踐魏良輔（約1526-約1586）主張「頭腹尾音畢勻」的特色。從魏良輔的倡導，到沈寵綏的建構，不止彰顯戲曲音韻史的學術意義，更體現崑腔纏綿悠遠、玉潤珠圓的度曲美學。

Shen Chongsui wrote Duqü xu zhi (1639) as Kunshan accent was particularly popular in drama world after the Wanli period of the Ming dynasty. “Kunshan shuimo accent” features the roundedness when opening the mouth and the slenderness when ending sounds. Taking the sounds from the central plains as the “body” and the Wu language as the method of “application,” Shen created the theory of “sanzi qiefa,” also known as “sanyin heqie.” The theory refers to spelling the sounds of three characters into one sound; the head of the sound comes from the upper character, the belly comes from the middle character, and the bottom comes from the lower character. Shen has his own rationales for the construction of “qiefa ji changfa.” Firstly, the methods of spelling sounds are categorized as three types. The first type describes the phonetically combined the cuts (qie) of three characters. The second type is about the combined cuts of the head and the belly sounds. The third type pertains to the combined cuts of the head and the bottom sounds without “yinzhuan” between them. Secondly, Shen’s theory emphasizes that it is recognizable to differentiate the phonetic components from the upper, the middle, and the lower characters. Under the same rhymer group, even though the belly and the bottom sounds are the same, it is still recognizable to tell if the initial of the phonetically commented character is voiced or voiceless via the head sound. The belly and the bottom sounds can help to tell if the phonetically commented character is dark or light tone. Thirdly, his comments about mnemonic chants for remembering initials, tonguing, and ending sounds are made to tell the differences of phonology between the central plains sound system and Wu language. These methods offered by Shen meant to teach Wu people for mastering sounds when singing Kunshan drama. “Sanzi qiefa,” provides convenient methods for singers to master the head (initials and medials), the belly (main vowels), and the bottom sounds (finals) without missing all important parts of syllables, which proves Shen’s idea about “qiefa ji changfa” and practices what Wei Liangfu advocated regarding “toufuweiyin bi yun.” What it had been done from Wei to Shen not only presents the academic achievement of the phonology of traditional Chinese opera, but also shows the aesthetics of Duqü in terms of the Kunshan features of lasting and mellowness as the sounds of rubbing water.