Wikipedia
The Ringvereine (English: "Ring clubs", as members identified themselves by wearing a ring) were criminal gangs operating in late 19th and early 20th century Germany, notably the Wiemar period. Ostensibly convicts associations formed in the 1890s to aid their reintegration to society, the Ringvereine became convenient fronts for illegal activities as well as providing a ready network of underworld contacts. Like the Mafia, these associations also followed certain rules and a code-of-conduct. Since these groups practiced witness intimidation and members provided alibis for one another, it was difficult to prove their involvement in a crime.
When the Nazis came to power in 1933 they promised to restore law and order and outlawed Ringvereine associations. Nevertheless, despite being banned and the best efforts of the Nazi regime, these organised gangs would persist for a number of years.