The popular poetry style at that time was mainly five-character poems, so they were all written in five-character poems.
As stated in "Wenjing Mifu Lun", the eight diseases are ancient terms for the rhythm of poetry. It was proposed by Liang Chenyue of the Southern Dynasty that the problems occurred in the use of the four tones.
It refers to the eight shortcomings that should be avoided in poetry, namely flat head, upper tail, wasp waist, crane knee, big rhyme, small rhyme, side button, and straight button. It has positive significance for increasing the beauty of the art form of poetry and enhancing the artistic effect of poetry, but it also brings certain disadvantages.
This marks that ancient Chinese poetry has moved from "ancient style" poetry, a product of primitive natural art, to "modern style poetry", a man-made art.
Extended information
The pros and cons of the "Eight Diseases"
Shen Yue's statement was criticized by Zhong Rong and others at the time. Song Yanyu's "Canglang Poetry Talk·Poetic Style" also said: "There is no need to stick to this when writing poetry, and there is no evidence for the bad methods." Contrast poetic terms. Refers to the duality of words and phrases in poetry. It can be relative between two sentences or within a sentence. Contrasts generally use the same sentence patterns and parts of speech.
As a metrical requirement, the middle two lines of the verse must be in opposition, but the first and last couplets do not need to be in opposition. But there are also variations, either the neck couplet is not in opposition, or the tail couplet is in opposition; it is less common that the first couplet is in opposition. There is no need for antithesis in quatrains, but there are occasional couplets. ? Work on poetic terminology.
The antithesis must use the same part of speech, such as noun to noun, pronoun to pronoun, adjective to adjective, adverb to lyrics, function word to function word. In the old days, nouns were divided into categories such as astronomy, seasons, geography, utensils, clothing, food and drink, stationery, literature, vegetation, birds, beasts, insects, fish, body, human affairs, and human relations.
Strict antitheses, parts of speech, and parts of speech must be relative, which is called work pair
Baidu Encyclopedia-Eight Diseases