The tandem bicycle or twin is a form of bicycle (occasionally a tricycle) designed to be ridden by more than one person. The term tandem 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 World War II until a revival started in the late 1960s. 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 McCready 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.
Compared to a conventional bicycle, a tandem has double the pedalling power, without necessarily doubling the speed, and with only slightly more frictional loss in the drivetrain. It has about the same wind resistance as a conventional bicycle. High-performance tandems may weigh less than twice as much as a single bike, so the power-to-weight ratio may be slightly better than that of a single bike and rider. On flat terrain and downhill, most of the power produced by cyclists is used to overcome wind resistance, so tandems can reach higher speeds than the same riders on single bicycles. They are not necessarily slower on climbs, but are perceived as such, in part due to the need for a high level of coordination between the riders, especially if the physical abilities of the two riders are very different, requiring a compromise on cadence.
On conventional tandems, the front rider steers as well as pedals the bicycle and is known as the captain, pilot, or steersman; the rear rider only pedals and is known as the stoker, navigator or rear admiral. On most tandems the two sets of cranks are mechanically linked by a timing chain and turn at the same rate. As time has moved on so has the use of 'captain' and 'stoker' as terms for riders of a tandem. As both are seen as riders the use of the words 'front rider' and 'rear rider' are far more descriptive to modern tandem riders.