The provision of social housing has a long history in France, and since 2000 is enshrined in law: local authorities in populous areas must aim to ensure that at least 20% of their housing stock is social, or face an annual fine. As well as guaranteeing a sufficient supply of affordable dwellings, this legislation was intended to promote what the French call mixité sociale (social diversity) and prevent, or reverse, the formation of ghettos of rich and poor.