Professional associations that regulate nearly every trade and activity in Ankh-Morpork, from the Assassins' Guild (which licenses and regulates murder for hire) to the Thieves' Guild (which provides a quota-based theft service) to the Seamstresses' Guild (which does not, in fact, primarily involve sewing). The Guild system is Lord Vetinari's method of maintaining order: by licensing and regulating activities that would happen anyway, he keeps them predictable and taxable.