The public transport system in Frankfurt is part of the Rhein-Main-Verkehrsverbund (abbreviated: RMV) transport network and consists of several carriers who all use the same fare system. Therefore, one ticket is valid for a journey which may include several modes of transit run by different operators. The fares are paid in advance of travel at a ticket vending machine or at the driver on board a bus. There are no turnstiles or other controlling barriers; instead, a proof-of-payment system is used. Plainclothes fare inspectors are employed and carry out random checks to ensure passengers have paid. If found to be travelling without a ticket, then they are required to pay a fine.