《文心雕龙·声律》
所有声音都有飞扬下沉两种，音响有双声叠韵两种；双声字中间被其他字隔开，就往往不协调，叠韵词分离在两处，就必然违背声律；一个句子的字声全是低沉的，声音就像要断气一样，全是高昂的，就一直上升而不婉转：应使低昂之声像转动辘轳一样相互交错，像鱼龙的鳞甲那样整齐排列；声律的适当配合稍有错乱，就会前阻后碍，这种毛病，就是文人文章佶屈聱牙。
凡声有飞沉，响有双叠。双声隔字而每舛，迭韵杂句而必睽；沉则响发而断，飞则声飏不还，并辘轳交往，逆鳞相比，迕其际会，则往蹇来连，其为疾病，亦文家之吃也。
There are two types of sounds, rising and falling; there are two types of sound combinations, double initials and double finals; when words with the same initial are separated by other words, they often sound jarring; when rhyme words appear in different places, this inevitably violates the principles of euphony; if all the words in a sentence sound low, the sentence sounds weak and gasping; if all the words sound high, the sentence sounds too strained and forceful. For the sounds in a sentence to be euphonious, they must rise and fall in an intricate way, like a spinning winch; their arrangements must be as uniform as the scales of a fish or dragon; if the euphony is even slightly discordant, the sounds will hinder one another; this shortcoming results in the pretentious obscurity of many learned and artistic writings.