《文心雕龙·声律》
古代教唱歌，首先要琢磨发音的方法，使疾呼合于宫音，徐呼合于徵音。
古之教歌，先揆以法，使疾呼中宫，徐呼中征。
In ancient times, to teach singing, one must first deliberate on the method of pronunciation, so that fast exhalation matches the "gong" tone, and slow exhalation matches the "zhi" tone.