Xiao is the respect and care of children for parents and of the young for the old; ci is the love and protection of the old for the young. Together they are what the generations of a household owe one another. Parents bring up their children, and when the children are grown they in turn support their parents, which is "feeding back".
Western Zhou bronze inscriptions often have phrases like "to offer filial service" to the ancestors, and at that time xiao was an ethic of sacrifice and reverence for forebears.
At the end of the Warring States period the Classic of Filial Piety was composed, raising xiao to a systematic ethic and calling it "the root of virtue".
Emperor Wu of Han had the commanderies recommend the filial and incorrupt, so that xiao became a criterion for office, and the Han thus claimed to rule the realm through filial piety.
Filial Piety is the class in the chapter on Values on the ethic between old and young in the family: mutual respect, and care repaid with gratitude. Western Zhou bronze inscriptions already read "to offer filial service" to the ancestors, the Classic of Filial Piety of the late Warring States called filial piety "the root of virtue", and Emperor Wu of Han had the commanderies recommend the filial and incorrupt, the Han claiming to rule the realm through filial piety.
