The first couplet, the parallel couplet, the neck couplet and the tail couplet are dedicated to metrical poems. They are composed of eight sentences, each of which is related. The first couplet to the fourth couplet are named after this, and the antithesis of metrical poems is roughly in the middle two couplets, namely, parallel couplet and neck couplet. The first couplet, the parallel couplet, the neck couplet and the tail couplet are similar to prose, and the pen starts at the beginning and closes. The first couplet is the beginning of the poem "Qi". They belong to Cheng, Zhuan and Hewei. The difference between metrical poems and quatrains is mainly in the number of words, four quatrains and eight quatrains. Quatrains, also called "truncated sentences", generally rhyme as one, two or four sentences (except special ones). The quatrains may or may not be antagonistic. And the neck couplet of metrical poetry must be a antithesis. No matter whether it is a metrical poem or a quatrain, it has the requirement of being flat and even. A metrical poem with more than eight sentences is called arrangement.