The sport of snooker has utilised a world rankings system since 1975, used to seed players on the World Snooker Tour for tournaments. Originally, rankings were published once a year at the conclusion of a season: this had the effect of ensuring the World Champion would be the top seed for the entirety of the subsequent season. Since 2010, however, the rankings were changed so that they would be updated after every ranking tournament.[1] The number one ranking has been held by twelve players; Ray Reardon was the first to hold the position, and was followed by Cliff Thorburn, Steve Davis, Stephen Hendry, John Higgins, Mark Williams, Ronnie O'Sullivan, Neil Robertson, Mark Selby, Judd Trump, Ding Junhui and Mark Allen.
Hendry held the number one position for the longest time under the annual format, holding it for nine years in total. Since it changed to a rolling format in 2010, Selby has held the rank longer than anyone else.