Under the imperial state, officials could not serve in their home region. This was the law of avoidance (回避). An official’s native place and the place he governed could not be the same or neighboring. Close relatives could not serve in the same office, or in offices directly above and below each other. The aim was to prevent favoritism and local protection.
Today’s Civil Servant Law keeps the principle. Civil servants who are spouses or close relatives may not work in posts where one directly supervises the other.