Why do ancient poems have six lines?

In the Tang Dynasty, metrical rules appeared in poetry, that is, some rules for writing poetry were gradually formed, such as rhyming (strictly speaking, the rhyme must be flat), antithesis, leveling and so on. It was stipulated that four lines were called quatrains, eight lines were called metrical poems, and five words were called five words in each line and seven words were called seven words. In addition, they were all unqualified. For example, six lines, or six words, do not meet the requirements.

These metrical rules require unification into the mainstream from the early Tang Dynasty to the prosperous Tang Dynasty, and to the middle and late Tang Dynasty.

However, before the Tang Dynasty, there was no such rule in writing poems. At first, all poems were written in four words. From the Book of Songs to Cao Cao's poems, there are four words (four words per line), and there is no limit on the number of lines. Until the prosperous Tang Dynasty, Li Bai, Du Fu and a large number of poems still ignored the rules.

Therefore, ancient poetry itself is not limited to the number of lines and words per line, but gradually evolved into a fixed number of lines and words.

The most famous of the six lines is:

A traveler's song

The thread in the loving mother's hand,

Make clothes for her wayward son.

She sewed it carefully, mended it thoroughly,

Afraid that the delay would make him come home late.

But how much love does an inch of grass have,

Make three spring scenery.

This poem is a famous sentence by Meng Jiao, a poet in the middle Tang Dynasty. It can be seen that in the middle Tang Dynasty, six-line ancient poems were still very common.