London has two types of taxis: the famous black cab, and so-called minicabs. Black cabs are the only ones licensed to pick people up off the street, while minicabs have to be pre-booked. The famous black cabs can be hailed from the curb or found at one of the many designated taxi ranks. Cabs that are free for business have their yellow TAXI light illuminated. In order to be licensed for a black cab, drivers must take ‘the knowledge’, an extensive exam in London's streets; this means they can, theoretically, navigate you to almost any London street without reference to a map.