Tandem (or in tandem) is an arrangement in which a team of machines, animals or people are lined up one behind another, all facing in the same direction.
The original use of the term in English was in tandem harness, which is used for two or more draft horses, or other draft animals, harnessed in a single line one behind another, as opposed to a pair, harnessed side by side, or a team of several pairs. The tandem harness allows additional animals to provide pulling power for a vehicle designed for a single animal.
Tandem seating may be used on a tandem bicycle where it is alternative to sociable seating. Tandem can also be used more generally to refer to any group of persons or objects working together, not necessarily in line.
In heavy trucks tandem refers to two closely spaced axles. Legally defined by the distance between the axles (up to 2.5m in the EU, 40” to 96” in the US), mechanically there are many configurations. Either or both axles may be powered, and often interact with each other. In the US both axles are typically powered and equalized, in the EU one axle is typically unpowered, and can often be adjusted to load, and even raised off the ground, turning a tandem into a single axle. 
The two seating configurations for trainer, night / all-weather interceptor or attack aircraft are pilot and instructor side by side or in tandem. Usually the pilot is in front and the instructor behind. In attack helicopters, sometimes the pilot sits in back with the weapons operator in front for better view to aim weapons, as the AH-1 Cobra was a tandem cockpit redesign which produced a much slimmer profile than the UH-1 Huey on which it was based. Attack aircraft and all-weather interceptors often use a second crew member to operate avionics such as radar, or as a second pilot. Bombers such as the B-58 Hustler seated 3 crew members in tandem. A common engineering adaptation is to lengthen the cockpit or fuselage to create a trainer with tandem seating from a single seater aircraft.
An alternative configuration is side-by-side seating which is common in civil aircraft of all sizes, trainers and large military aircraft but less so in high performance jets and gliders where drag reduction is paramount. The B-47 and XB-52 bombers used fighter-style tandem seating but the final B-52 bomber series used a conventional side-by-side cockpit. The Grumman A-6 Intruder, General Dynamics F-111 or the Sukhoi Su-24 are examples of combat aircraft which use this configuration. For training aircraft, it has the advantage that pilot and instructor can see each other's actions, allowing the pilot to learn from the instructor and the instructor to correct the student pilot. The tandem configuration has the advantage of being closer to the normal working environment that a fast jet pilot is likely to encounter.
In some cases such as the EA-6B Prowler, a two-place aircraft can be modified to be made longer into a four-place aircraft. Also a single seat cockpit can be redesigned into a side-by-side arrangement in the case of the A-1 Skyraider, TF-102 trainer or the Hawker Hunter training versions.
- Tandem language learning
- Class 4 telephone switch or tandem switch
- Semi-trailer, where multiple axles on a trailer are called "tandem axles".
- In series, an arrangement of electrical circuit elements one after another.
- Tandem-charge, an arrangement of two explosive charges one behind another in a projectile.
- Tandem skydiving, two people skydive using one parachute
- Tandem bicycles
- Tandem product design
- "Tandem". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005.
- "Tandem". Wordinfo.com. Retrieved 2009-01-28.
- "Guidelines on Maximum Weights…Criteria". Road Safety Authority. 2013. Retrieved 2013-05-20.
- Federal Highway Administration (2006). "Freight Management and Operations: Bridge Formula Weights". U.S. Department of Transportation. Retrieved 2013-05-20.
- "Why Tandem Seating in the SGT-300?". testrakeaviation. 2010-03-15.
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- Clipart.com Closeup | Royalty-Free Image of tandem carriage horses harness coach driver whip Illustration.
- Three Men In a Carriage Drawn by a Tandem Horse Team. Tandem Horse Teams Museum Victoria (Australia) official site; illustration.