The IAAF keeps world records for five different types of track relays. As with 4×100 m and 4×400 m events, all races comprise teams of four athletes running the same distances, with the less commonly contested distances being the 4×200 m, 4×800 m and 4×1500 m relays. Other events include the distance medley relay (comprising legs of 1200 m, 400 m, 800 m, and 1600 m), which is frequently held in the United States, and a sprint relay, known as the Swedish medley relay, which is popular in Scandinavia and was held at the IAAF World Youth Championships in Athletics programme. Relay events have significant participation in the United States, where a number of large meetings (or relay carnivals) are focused almost exclusively on relay events.