This concept refers to the four tones (the level tone, falling-rising tone, falling tone and checked tone) and the eight rhythmical errors. The conscious use of these tones and the avoidance of these errors were metrical norms for poetry creation proposed by the literati of the Qi and Liang reigns of the Southern Dynasties, such as Shen Yue ( 441-513 ). Chinese tones in even earlier times remain controversial in academic circles. By the time of Qi and Liang, however, Zhou Yong, Shen Yue and others for the first time defined the four tones of the Chinese language - the level tone, falling-rising tone, falling tone, and checked tone. In addition, Shen Yue proposed pairing the five sounds with the four tones. Thereafter, the literati have actively used them in their poetry to perfect the sound and mood. Shen also found that improper use of the four tones would lead to eight rhythmical errors, such as pingtou (平头overlapping tones at the beginning), shangwei (上尾repeated tonesat the end), fengyao (蜂腰 the use of characters with the same tone which makes the poem resemble what he likened to a wasp's waist), and hexi (鹤膝 the use of characters with the same tone which makes the poem resemble a crane's knee). These errors in poetry should be avoided as much as possible. “Overlapping tones at the beginning" refers to the first and the sixth characters or the second and the seventh characters in two related lines in a five-character poem having overlapping tones. This was considered to have violated the rules. “Repeated tone at the end" refers to the fifth and the tenth characters in two related lines of a five-character poem having the same tone. Several other errors list similar rhythmical problems.The concept of four tones and eight rhythmical errors was a major advancement in the theory of Chinese phonology, but the concept was too restrictive to use in practice. Therefore, it was opposed by many people at the time because they believed that it hindered the natural beauty of rhythms and rhymes in poetry creation. In the Tang Dynasty, the literati refined the concept to form the theory of level and oblique tones, cutting the requirement of four tones to a requirement of two, making the composition of poems easier, thus greatly improving the rules and forms for the so-called modern-style classical poetry. Among these people Du Fu's poetry creation is a perfect model.

At that time, it was popular to compose poetry. Shen Yue from Wuxing Prefecture, Xie Tiao from Chen Prefecture, and Wang Rong from Langya Prefecture shared the same interest in poetry and admired each other. Zhou Yong from Runan Prefecture was a master of tones and rhythms. The poems of Shen Yue and others used the temperament of gong and shang and determined the metrical structure with the level tone, falling-rising tone, falling tone, and checked tone, while Shen Yue also pointed out the errors of overlapping tones at the beginning, repeating tones at the end, the use of characters with the same tone which makes the poem resemble a wasp waist, and the use of characters with the same tone which makes the poem resemble the knee of a crane. The rhymes of the five characters in a line of a poem are different, the rhythms for jue and zhi within two lines of a poem should vary, and the rules should not be altered. The people of the time called the style of poetry they created the Yongming Style. (The History of the Southern Dynasties)

If five-character poems are well composed using the four tones, they can be sung by the people and become widely popular; if they are composed with care to avoid the eight rhythmical errors, they can become exquisite and polished. (Kūkai, Japan: On Chinese Poetry)