ACE Routemaster bus in London.
A double-decker bus is a bus that has two stories or decks. Red double-decker buses are used for mass transit in London. Double-decker buses are also used in other cities in Europe, Asia, and former British Colonies and protectorates such as Hong Kong and Singapore.
Almost all double-deckers have a single, rigid chassis.
This type of bus is often used for touring rather than for mass transit. As William Ewart Gladstone observed, "...the way to see London is from the top of a 'bus". (Gladstone was referring to London's once ubiquitous double-deck horse drawn omnibuses, rather than a motor bus).