Whereas athletes in other sports are free to wear the jersey number of their choice, football players are required to wear certain numbers based on their positions. While restrictions on jersey numbers do not apply at the college and youth levels, players participating at the high school or professional level (NFL) must abide by the range of numbers assigned to each position.
The NFL first implemented jersey number regulations in 1973. The list was initially very strict and did not give players many different numbers to choose from, leading to slight modifications in 2004 that resulted in the current list of permissible jersey numbers.
Included in the list of jersey numbers that players are not permitted to wear are retired numbers. Often, teams will choose to retire the numbers worn by legendary players that achieved great success throughout their careers (many are also inducted into the Pro Football Hall of Fame) such that no player on that team can wear that number again. Having a retired number is one of the greatest honors a player can receive, as it indicates that the player reached an unmatchable level of success. Among the list of retired numbers for several teams throughout the league are 8 (Pat Tillman, Arizona Cardinals), 80 (Jerry Rice, San Francisco 49ers), 55 (Derrick Brooks, Tampa Bay Buccaneers), 75 (Joe Greene, Pittsburgh Steelers) and 1 (Warren Moon, Tennessee Titans/Houston Oilers).