tours have been contested over the years. A two-tier structure was adopted for the 1997–98 snooker season; comprising six tournaments known as the WPBSA Minor Tour was open to all professionals, but only ran for one season. A similar secondary UK Tour was first played from the 1997–98 season, which was renamed the Challenge Tour in 2000, Players Tour Championship in 2010 and returned as the Challenge Tour in 2018. The global governing body for professional snooker is the World Professional Billiards and Snooker Association (WPBSA), founded as the Professional Billiard Players' Association. The WPBSA owns and publishes the official rules of snooker, and has overall responsibility for policy-making in the professional sport of snooker. World Snooker Ltd responsible for the professional tour which is owned by both the WPBSA and Matchroom Sport. World rankings Every player on the World Snooker Tour is assigned a position on the WPBSA's official world ranking list, which is used to determine the seedings and the level of qualification each player requires for the tournaments on the professional circuit. The current world rankings are determined using a two-year rolling points system, where points are allocated to the players according to the prize money earned at designated tournaments. This "rolling" list is maintained and updated throughout the season, with points from tournaments played in the current season replacing points earned from the corresponding tournaments of two seasons ago. Additionally, "one-year" and "two-year" ranking lists are compiled at the end of every season, after the World Championship; these year-end lists are used for pre-qualification at certain tournaments and for tour-card guarantees. The top 16 players in the world ranking list, generally regarded as the "elite" of the professional snooker circuit, are not required to pre-qualify for some of the tournaments, such as the Shanghai Masters, the Masters and the World Snooker Championship. Certain other events, such as those in the Players Series, use the one-year ranking list to qualify; these use the results of the current season to denote participants. As of the 2020–21 season, there are 128 places available on the World Snooker Tour, with players either in the top 64 on the official ranking list, or finishing as one of the top eight prize money earners during the most recent season, guaranteed a tour place for the next season, this being assessed after the World Championship. Tournaments The oldest current professional snooker tournament is