In London, hackney-carriage drivers have to pass a test called The Knowledge to demonstrate that they have an intimate knowledge of the geography of London streets, important buildings etc. There are two types of badge, a yellow one for the suburban areas and a green one for all of London. The latter is considered far more difficult. Drivers who own their cabs as opposed to renting from a garage are known as “mushers” and those who have just passed the “knowledge” are known as “butter boys”.[citation needed] There are currently around 21,000 black cabs in London, licensed by the Public Carriage Office.[13]