HangZhou OEM Bicycle Industrial Co., Ltd
The tandem bicycle or twin is a form of bicycle (occasionally, a Tricycle) designed to be ridden by more than one person. The termtandem refers to the seating arrangement (fore to aft, not side by side), not the number of riders. A bike with two riders side by side is called a sociable.
Patents related to tandem bicycles date from the late 1890s.In approximately 1898, Mikael Pedersen developed a two-rider tandem version of his Pedersen bicycle that weighed 24 pounds, and a four-rider, or "quad", that weighed 64 pounds. They were also used in the Second Anglo-Boer War. Tandem popularity began to decline after WWII until a revival started in the late sixties. In the UK The Tandem Club was founded in 1971, new tandems came on to the market from the French companies Lejeune and Gitane, and in the USA Bill McReady founded Santana Cycles in 1976. Modern technology has improved component and frame designs, and many tandems are as well-built as modern high-end road and off-road bikes.
Tandems can have more than 2 riders - tandem refers to the arrangement of the riders one behind the other rather than the number of riders. Bicycles for three, four, or five riders are referred to as "triples" or "triplets", "quads" or "quadruplets", and "quints" or "quintuplets" respectively. One such familiar to UK TV viewers was the "trandem" ridden by The Goodies. Originally a 2-man tandem with an extra "dummy" seat attached, a full 3-man version was built for them by Raleigh.
Some designs such as the DaVinci can allow independent pedaling through the use of multiple freewheels.  In another design, the rear rider steers and propels the rear wheel with pedals, and the front rider propels the front wheel with both hands and feet.