The rhyme book used in the Tang Dynasty is Qieyun written by Sui Lu Fa Yan, and it is also the originator of all rhyme books in later generations. Song people added Qieyun to Guang Yun, with more than 200 rhymes. In fact, the rhymes used by poets in Tang and Song Dynasties are not entirely based on these two rhymes. Poets' rhymes in Tang and Song Dynasties are well reflected in Ping Yun Shui edited by Wang Jinwen Yu, and later poets' rhymes are generally based on Ping Yun Shui. "Ping Yun Shui" has 106 rhymes, of which Pingsheng has 30 rhymes, which are divided into upper and lower halves, called upper and lower Pingsheng. This is just for the convenience of arrangement, and there is no difference in tone between the two. Modern poetry only pays attention to rhyme, so let's just look at the rhyme of each part of these rhymes (the first word of each rhyme): Shang Pingsheng: Dong Yi, Erdong, Sanjiang, Sizhi, tachileik, Liu Yu, Chyi Yu, Eight Banners, Jiujia, Shi Hui, Shizhen, 12, 13 yuan. Some of them belonged to different rhymes in ancient times, but now they can't tell the difference, such as East and Winter, River and Yang, Fish and Danger, True and Wen, Xiao, Yao and Hao, Salty and Salty, Geng and Qing, Cold and Delete, and so on. If we look at the words in each rhyme, we will find the opposite situation: what the ancients thought belonged to the same rhyme does not rhyme at all today. The rhyme of classical poetry can be mixed with the rhyme of neighboring rhymes, such as Dong Yi and Er Dong, Si Zhi and Wu Wei, which are called tongyun. However, the rhyme of modern poetry must strictly only use words with the same rhyme. Even if the number of words in this rhyme is small (called narrow rhyme), it cannot be mixed with words in other rhymes. Otherwise, it is called rhyme, which is the taboo of modern poetry. But if the first sentence rhymes, you can borrow adjacent rhymes. Because the first sentence can be put or not, you can bend the rules.
Modern people can of course write modern poems with modern rhymes. If we want to use Pingyunshui according to tradition, we must pay attention to the differences between ancient and modern sounds, especially when reading ancient poems. These differences can sometimes be distinguished by dialects, but they are not necessarily reliable, so we should read more and recite more.