There are a number of competitions swum regularly in medley relay, both by men and women. Mixed-gendered medley relays were introduced at the 2014 FINA World Swimming Championships (25 m) (4×50 m) and 2015 World Aquatics Championships (4×100 m). The event will debut at the 2020 Summer Olympics (4 × 100). Standard United States high school swim meets have short course events, that is the lengths are typically swum in a 25-yard or meter long pool. One relay event swum in State or Sectional Championships is the 4×50 yard medley relay. Many collegiate programs hold competition in the 4×50 medley relay, and 4×100 medley relay. The technique for medley relay events does not differ much from the technique for the separate events for the four strokes. The first swimmer swims the backstroke normally. The only difference for the following swimmers is that there is no start signal, but rather the previous swimmer completing his or her turn by touching the wall signals the start for the subsequent swimmer. It is very important for the next swimmer off the block to accurately judge the time at which the swimmer in the water will touch the wall. A fast reaction could result in a significant time gain in the race, but a false start (diving early) will result in a disqualification. FINA rules require that a foot of the second, third or fourth swimmer must be contacting the platform while (and before) the incoming teammate is touching the wall; the starting swimmer may already be in motion, however, which saves 0.6–1 seconds compared to a regular start. Furthermore, many swimmers may perform better in a relay than in an individual race owing to a team spirit atmosphere. As a result, relay times are typically 2–3 second faster than the sum of best times of individual swimmers. Prior to 1952, the butterfly was not defined as a separate stroke from the breaststroke, and so medley races featured only three styles: backstroke, breaststroke, and freestyle. The usual distance of both the IM and the medley relay was thus 300 metres or yards rather than 400. In the United States, during the year of 1953, some medley races included the butterfly stroke, and the Amateur Athletic Union made it mandatory from 1954. These are the official rules of the FINA regarding medley swimming:   Freestyle includes a special regulation for medley events:   Additionally, the normal rules of relay events apply:   Men Women  Mixed