Four musts: evolved from metrical poems. The structure of seven-character quatrains is four sentences in the whole poem, with seven words in each sentence. Four sentences rhyme, two or four sentences rhyme, and the first sentence may or may not rhyme. There are four flat formats:

That is, there are four kinds: the first sentence rhymes, the first sentence rhymes, the first sentence rhymes, and the first sentence rhymes.

Pinglv originated from the "double-headed change" before the Tang Dynasty. The main rule is that in a metrical poem, "Pingsheng" and "Xusheng" appear alternately, while in two antitheses, "Pingsheng" and "Xusheng" are opposite. The couplet above and the antithesis below should be pasted together. For example, the first sentence of the seven-character rhyme is "flat and parallel" (flat and parallel alternate in this sentence), and the second sentence is "flat and parallel", that is, both the first sentence and the second sentence are flat and parallel. The third sentence must begin with "Pingping" in order to be consistent with the previous sentence. In addition, metrical poems also pay attention to whether the first sentence begins with two flat lines or two oblique lines. The former is called "flat rise" and the latter is called "inclined rise". In addition, people can flexibly master the use of flat tones and sounds when writing metrical poems. The general rule is that "no matter one, three, five, two, four and six are distinct", which means that in a sentence, the first, three and five words can be flush, while the second, four and six words are rhythmic points, so they are.