Relocation of professional sports teams occurs when a team owner moves a team, generally from one metropolitan area to another, but occasionally between municipalities in the same conurbation. The practice is most common in North America, where a league franchise system is used and the teams are overwhelmingly privately owned. Owners who move a team generally do so seeking better profits, facilities, fan support, or a combination of these. North America Unlike most professional sport systems worldwide, North America does not have comprehensive governing bodies whose authority extends from the amateur to the highest levels of a given sport. North American sports generally do not operate a system of promotion and relegation in which poorly performing teams are replaced with teams that do well in lower-level leagues. A city wishing to get a team in a major professional sports league can wait for the league to expand and award new franchises. However, such expansions are infrequent, and generally limited to a narrow window in time. Many current owners believe 32 is the optimal size for a major league due to playoff structure and ease of scheduling. As of 2018, each of the major leagues has between 30 and 32 franchises. The National Hockey League (NHL) has expanded to 32 teams, with the Vegas Golden Knights having become the league's 31st team in 2017 and the Seattle Kraken becoming the 32nd team in 2021. In past decades, aspiring owners whose overtures had been rejected by the established leagues would respond by forming a rival league in hopes that the existing major league would eventually agree to a merger; the new league would attain major league status in its own right; or the established league was compelled to expand. The 1960s American Football League (AFL) is perhaps the most recent example of a successful rival league, having achieved each of the three goals listed above in reverse order. However, all major sports have had a rival league achieve at least some of these goals in the last half of the 20th century. Baseball's proposed Continental League did not play a game but only because Major League Baseball responded to the proposal by adding teams in some of the new league's proposed cities. The American Basketball Association (ABA) and World Hockey Association (WHA) each succeeded in getting some of their franchises accepted into the established leagues, which had both unsuccessfully attempted to cause