Under Wang Anshi's New Policies around 1070 the Song state organised households into a nested baojia hierarchy, typically ten households to a jia and ten jia to a bao of about a hundred households, each with an appointed head. The heads kept population rosters, watched conduct, raised local militia and shared collective liability for a member's crime or unpaid tax. By making neighbours answerable for one another it pushed registration, policing and corvée down to the street and village without enlarging the bureaucracy. Revived by later dynasties and into the twentieth century, baojia knitted surveillance into the residential grid.
Baojia tied household registration, taxation and policing to the spatial unit of the neighbourhood, an early template for the registration-based control of urban populations that the hukou later perfected.