The so-called "four-character verse" of Book o f Songs is essentially a "two two-four" sound-rhythm structure. Its molding influence covered poetry, music and dance during the Zhou Dynasty, a period of "absolute molding" of the four-character structure of Chinese. The regular sentence pattern of Da Ya, that is, a paragon of the style of Book o f Songs, actually reflected the rhythm of court music under the restriction of Zhou double-tone bells. The rhythm, in turn, originated from the rhythmic framework of Qizhou (a period preceding the founding of Zhou Dynasty) folk songs shaped by the rhythm of two men tilling side by side (known as ougeng, a farming practice invented in the reign of King Wen). The continuation of the style of Book of Songs ultimately hinged on the effectiveness of the Square-Fields System, which was based on the Zhou si, a spade like farm tool made of wood. Hence, the advent of the Chinese Iron Age in the early Warring States Period led to the collapse of the Square-Fields System, the loss of court music, and the disappearance of the style of Book o f Songs, thereby bringing an end to the period of the "absolute molding" of the four-character structure of Chinese.