Per 《
新华成语词典》, derived from 《
白虎通·
三纲六纪》 (Eastern Han) and
王充's 《
论衡》. The
三纲 ('three bonds'):
君为臣纲 (ruler-subject),
父为子纲 (father-son),
夫为妻纲 (husband-wife). The
五常 ('five constants'):
仁 benevolence,
义 righteousness,
礼 propriety,
智 wisdom,
信 faithfulness. Fixed by
董仲舒 in the Han, these defined the moral software of imperial China for two millennia.