I believe it would be the Jim crow law
The jim crow law is a set of regulations that is aimed to racially segregate the infrastructure that can be used by black and white citizens after the reconstruction.
The laws prohibit blacks citizens to use the same schools, public transportation, restaurants, or hospital as the white citizens.
Since the infrastructures that can be used by black citizens typically lower in quality, it really limited the chance that black people have to advance and prosper in the society.