The three sports combined in a standard triathlon are swimming, cycling, and running.
Athletes complete the three disciplines consecutively, with the clock continuing through the transitions between them. The first transition, called T1, is from swimming to cycling; the second, T2, is from cycling to running. Because competitors must change equipment quickly while already fatigued, transitions are often described as the sport’s “fourth discipline.”
The standard Olympic-distance triathlon covers a 1.5-kilometre swim, a 40-kilometre bicycle ride, and a 10-kilometre run. Other formats use different distances, but the swim-bike-run sequence remains the defining combination for a conventional triathlon. Indoor versions may use a pool, stationary bikes, and a treadmill.
The modern sport developed prominently in Southern California during the 1970s, although earlier three-sport events existed in France. Triathlon became an Olympic sport at the 2000 Sydney Games. Related events can cause confusion: a duathlon combines cycling and running, while an aquathlon combines swimming and running.