The ancients said that articles have four tones and eight diseases. What exactly do they mean?

Shen Yue, a poet in the Southern Dynasties, put forward a theory about the rhythm of poetry. "Four tones" refers to the names of various tones represented by the words "Ping, Shang, Qu and Ru". "Eight diseases" refer to eight disadvantages that should be avoided in poetry, namely, flat head, tail, bee waist, crane knee, big rhyme, small rhyme, side button and straight button. The fifth word at the top of the finger must not be synchronized with the cross (rhyme can be ignored). The second word in the five-character poem refers to the waist of a bee. The fifth word in the five-character poem refers to the knee of the crane. The fifth word in the five-character poem refers to the knee of the crane. If the rhyme of a five-character poem is "new", "person", "Tianjin", "neighbor" and "style", it should not be more secure. Xiaoyun refers to the overlapping crime except rhyme (that is, the nine-character mutual crime). There is a big button in Fat Girl, that is, there is the word "moon" in five sentences, so don't add the words "fish", "yuan", "Ruan" and "wish". There is a small button in the bull, which is "Ren" and "?" The words "benevolence" and "gold" are a button, and the word "benevolence" already exists in five words and one sentence, so it can no longer be safe. The words "Ren" and "Ru" lead to the disease of four tones. His theory is complicated, which sets many unnecessary taboos for poetry creation in terms of melody. However, it has a certain influence on the transformation of five-character ancient poetry into rhythmic poetry.