Licensed crime in Ankh-Morpork

'Ankh-Morpork's enviable system of licensed criminals owes much to the current Patrician, Lord Vetinari. He reasoned that the only way to police a city of a million inhabitants was to recognise the various gangs and robber guilds, give them professional status, invite the leaders to large dinners, allow an acceptable level of street crime and then make the guild leaders responsible for enforcing it, on pain of being stripped of their new civic honours along with large areas of their skin. It worked. Criminals, it turned out, made a very good police force; unauthorised robbers soon found, for example, that instead of a night in the cells they could now expect an eternity at the bottom of the river.

However, there was the problem of apportioning the crime statistics, and so there arose a complex system of annual budgeting, chits and allowances to see that a) the members could make a reasonable living and b) no citizen was robbed or assaulted more than an agreed number of times. Many foresighted citizens in fact arranged to get an acceptable minimum of theft, assault, etc, over at the beginning of the financial year, often in the privacy and comfort of their own homes, and thus be able to walk the streets quite safely for the rest of the year. It all ticked over extremely peacefully and efficiently, demonstrating once again that compared to the Patrician of Ankh, Machiavelli could not have run a whelk stall'

- Terry Pratchett, Wyrd Sisters, 1988