Ping and Qi are the tones used in Chinese poetry. Flat means straight, oblique means curved. According to the rhyme books revised from the Sui Dynasty to the Song Dynasty, such as "Qie Yun" and "Guang Yun", there are four tones in Medieval Chinese, called Ping, Shang, Qu, and Ru. Except for the flat tone, the other three tones have changes in pitch, so they are collectively called oblique tones. There is a certain pattern for the use of oblique words in poetry, which is called meter. Ping tones and oblique tones refer to the rhythm of poetry composed of oblique and oblique tones. Pingchi is an attempt to dualize the four tones. The four tones are the four tones in ancient Chinese. The so-called tone refers to the pitch, rise and fall, and length of the voice. Ping and Zhi are summarized using incomplete induction method based on the four tones. Ping means straight and Zhi means twists and turns.
The ascending tone, the descending tone, the entering tone are flat, and the rest are flat tones.
In Mandarin, the entering tone disappears, and the entering tone is classified into the upper two tones of the oblique tone and the yinping and yangping of the flat tone. This leads to errors in judging the oblique tone of poems in Mandarin.