《庄子·杂篇·徐无鬼》
于是当着大家调整好瑟弦，放一张瑟在堂上，放一张瑟在内室，弹奏起这张瑟的宫音而那张瑟的宫音也随之应合，弹奏那张瑟的角音而这张瑟的角音也随之应合，调类相同的缘故啊。
于是乎为之调瑟，废一于堂，废一于室，鼓宫宫动，鼓角角动，音律同矣！
So while everyone was watching, he tuned a se, put one se in the hall and the other one in the inner chamber, played the gong notes on one se and the gong notes of the other one responded, played the jiao notes on one se and the jiao notes of the other one responded, because they were of the same tune.