As the five most fundamental interpersonal relationships in ancient China, ''Five Cardinal Relationships'' generalize and conceptualize the human relationship of the ancient Chinese society. Ruler-Subject, Brother-Brother and Friend-Friend are three sets of relationships among ''Five Cardinal Relationships.'' In the historical context in which The Book of Songs was compiled, these three sets of relationships seemed independent of each other but were actually closely-knit. According to historical documents, ''Three Jian Rebellion'' occurred after King Wu of the Zhou Kingdom overthrew the Shang Dynasty. The duke of Zhou crushed the rebellion and began to implement the system of enfeoffment, which exerted a profound effect on the human relationship at that historical period. The system established the status of the king of Zhou and the hierarchical order of political and economic rights through carrying out the economic management by granting fiefs to nobles and collecting taxes and tributes from them. Under such an enfeoffment system, the fiefs were normally bestowed on the ruler's kinsmen, as is pointed out in Zuozhuan, ''They all invested their full brothers with the rule of States.'' Therefore, according to historical documents, it is most likely that the king's brothers became the king's subjects by means of enfeoffment. Furthermore, Bronze Inscriptions provide direct proof of the parallel relation between Ruler-Subject and Brother-Brother in the Zhou's family.  By counting the word frequency of you (friend) in phrases such as peng you (friend), liang peng (good friend), you (friend), you sheng (friend-student) and you jun zi (noble friend) in The Book of Songs, it can be found that you appears most frequently in Ya. (Minor Odes of the Kingdom and Greater Odes of the Kingdom) The analysis of ''Ka loh,'' ''Ke tsuy'' and ''Yih,'' three Odes from Greater Odes of the Kingdom indicates that Friend-Friend relationship in these odes implies a Ruler-Subject relationship. Both bronze inscriptions and historical documents provide the evidence that as an ethical norm, you served as the behavior code of brothers so that the relationship of Friend-Friend comprised Brother-Brother relationship at that time and thereby Ruler-Subject was tied to Friend-Friend through Brother-Brother. Initially you was used as a norm to regulate the Brother-Brother relationship. Since Ruler-Subject and Friend-Friend turned out to be interchangeable in some odes in The Book of Songs, you evolved into you ji (friend) to regulate the Ruler-Subject relationship. The trace of you ji is detectable in subsequent works such as The Analects of Confucius which equate the Ruler-Subject relationship with the Friend-Friend relationship. The essence of you ji in The Book of Songs can be summarized as follows: The ruler should do good to his friend and the subject should assist the ruler to ''behave with reverent mien.'' Such behavior codes also impacted on the Subject-Subject relationship.