The characters Bo (伯), Meng (孟), Zhong (仲), Shu (叔), and Ji (季) are originally ordinals used in courtesy names to indicate a person's rank among his or her siblings of the same gender who survived to adulthood.
The eldest brother's courtesy name would be prefixed with the word "Bo" (or "Meng" if he was born to a secondary wife), the second with "Zhong", the youngest with "Ji", and the rest with "Shu".
As the power of the Three Huan became hereditary, the descendants of Duke Zhuang's brothers used the ordinal numbers as family names to distinguish their branches of the House of Ji.
While the oldest son by his main wife Wen Jiang became the heir, and subsequently Duke Zhuang of Lu, his other three sons - respectively Qingfu (慶父), Shuya (叔牙) and Jiyou (季友) all became important officials during the reign of Duke Zhuang, and gained substantial power in the state.
Jiyou then returned from his exile in the state of Qi with the younger brother of Duke Min, who then ruled as Duke Xi of Lu; while Qingfu was forced to commit suicide, Jiyou became the chief minister of Lu, a post that he held for 16 years and which was secured with the backing of the powerful state of Qi.