Poems usually limit the number of words in each sentence according to the style of the poem. China's earliest poems were structured with metrical poems, and the metrical requirements were strict. For example, the poems in the pre-Qin period were generally four words per sentence, which were found in the Book of Songs. Later, it developed into a five-character or seven-character rhythmic poem, which was found in Tang poetry. After the further development of economy and culture in Song and Yuan Dynasties, the content of poems was gradually expanded and deduced. In the late period of the new-democratic revolution, poems evolved into free-style poems without the limitation of words.