China's poems are always accompanied by music, including ritual music and folk songs, which are all used for singing. The Book of Songs is also a music song with tunes. The tunes of The Book of Songs are mainly in the form of sentences, and there are words suitable for repeated singing. It's a little Song Like.
It is precisely because poetry and music are inseparable that when it became an independent art form of "poetry", it began to pay attention to the "phonology" of the words themselves. With the development of phonology, poetry has gradually moved towards the road of paying attention to flat and even tones. This is a gradual process, and it was not until the Tang Dynasty that strict rules were formed.