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A double-deck aircraft has two decks for passengers; the second deck may be only a partial deck, and may be above or below the main deck. Most commercial aircraft have one passenger deck and one cargo deck for luggage and ULD containers, but only a few have two decks for passengers, typically above a third deck for cargo.
Many early flying boat airliners, such as the Boeing 314 Clipper and Short Sandringham, had two decks. Following World War II the Stratocruiser, a partially double-decked derivative of the B-29 Superfortress, became popular with airlines around the world.
The first full double-deck aircraft was the French Breguet Deux-Ponts. The first partial double-deck jet airliner was the widebody Boeing 747, with the top deck smaller than the main deck. Boeing originally designed the distinctive 747 bubble top with air cargo usage in mind. The small top deck permitted the cockpit and a few passengers and nose doors with unobstructed access to the full length of the hold. Most 747s are passenger jets, and a small percentage are cargo jets with nose doors.
The first full double-deck jet airliner is the Airbus A380, which has two passenger decks extending the full length of the fuselage, as well as a full-length lower third deck for cargo. It entered service in October 2007. 
List of double-deck aircraft
- Double-deck flying boats
- Boeing 314 Clipper
- Dornier Do-X
- Short Sandringham
- Short Empire C-Class and the related G-class
- Saunders-Roe Princess - did not enter service.
- Partial second passenger deck
- Airbus A330 and Airbus A340
- Optional lower deck lavatories and crew rest
- Boeing 377 Stratocruiser
- Lower deck could be configured for lounge areas or additional seating
- Boeing 747
- Partial upper deck lounge areas or seating
- Optional upper deck crew rest and galleys
- Boeing 767
- Optional lower level crew rest area sleeps six
- Boeing 777
- Optional lower deck lavatories and galley
- Optional upper deck crew rest
- Junkers G.38
- Ilyushin Il-86
- Lower deck galley
- Lower deck "self loading luggage storage"
- Lockheed L-1011 Tristar
- McDonnell Douglas DC-10
- Lower deck galleys
- Tupolev Tu-114
- Full second passenger deck
- Cargo aircraft with a separate passenger deck
- Antonov An-225
- Antonov An-124
- Lockheed C-5 Galaxy
- Boeing C-97 Stratofreighter
- Douglas C-124 Globemaster II
- Short Belfast
- Lockheed R6V Constitution
- Blackburn Beverley - military transport, the main deck could be used for cargo or troops
- Double-deck cargo aircraft
- Aviation Traders Carvair
- Armstrong Whitworth AW.660 Argosy
- Bristol Freighter
- Convair XC-99
- Douglas C-124 Globemaster II
- Proposed double-deck passenger aircraft
- "Bed Down on the 'Bus: Jumbo Jets Going to the Mattresses". New York Post. Post Wire Services. October 16, 2007. Archived from the original on October 19, 2007.
- "Case Studies Crew Rest Module". Timco Aerosystems. Archived from the original on July 5, 2011.