How to understand the rhyme in poetry

Classical poetry rhymes in broad sense and narrow sense. In a narrow sense, rhyme refers to the use of the same "rhyme foot" in some sentence patterns, such as metrical poems, and the rhyme foot of the last word in every two sentences is the same. The so-called rhyme, according to modern Mandarin, is the last word of two or more sentences in a certain sentence pattern, and its vowels should be the same. However, in ancient times, because there was no unified pinyin for every word, and there were many dialects in different places, even in the same place, sometimes the pronunciation of the same word was different in different dynasties, so it was very complicated to unify the phonology with "rhyme book". My point is: there is no need to study it in depth now, just rhyme according to pinyin letters. Broadly speaking, rhyme means that every sentence in a poem is about "Ping" (except ancient style and Yuefu). The so-called flat tone can also be simply illustrated by the pinyin letters of modern Mandarin. Flat is the pronunciation of yin and yang, and the so-called flat tone is the pronunciation of the third and fourth tones (up and down). In ancient rhyme books, besides Yin Ping, Yang Ping, Shangsheng and Desheng, there is also a "Rusheng". This entering tone is generally a relatively short pronunciation, and there are both yin and yang in modern Mandarin, which is also very complicated. There is no need to study. Why do classical poems rhyme? Because classical poetry can be sung, especially words, which are usually sung with music and have the same rhyme, it is smooth to sing and easy to remember; If you add a level tone, the pronunciation of each poem will rise and fall, euphemistically. If there is no level tone, it may be shouting at the top of one's lungs, or it may have been dizzy all the time, or it may be "eight degrees higher" or even more than eight degrees, so it is not a poem. Therefore, the singing method of China's classical poems is different from the "reciting poems" of modern poems, which is not only beautiful in language, but also beautiful in phonology, which is beyond what foreigners call "poetry".