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”. There are currently around 21,000 black cabs in London, licensed by the Public Carriage Office.