In American football, uniform numbers are displayed on both the front and back of the jersey, and in many cases the sleeves, shoulder pad, or occasionally helmets. The numbers on the front and back are very large, covering most of the jersey. Certain numbers may only be worn by players in specific positions, thus assisting the officials in determining penalties. In two of the three levels of American football (high school football and college football), players must wear a number from 0 to 99. The number 0 is currently banned in professional football. Players who wear numbers from 50 to 79 are, by rule, prohibited from catching or touching forward passes if their team is in possession of the ball and may not line up in a position that allows them to do so, unless explicitly indicated to the referee during a tackle-eligible play. Other than this, the correspondence between jersey numbers and player positions is largely a matter of style, tradition and semantics. Canadian football follows a similar numbering scheme to that of American football, except that the ineligible numbers span only 50 to 69 and numbers 0 and 00 are available for use. Professional football The National Football League numbering system dates from a large-scale change of their rules in 1973, subsequently amended in various minor ways. As of 2021, players are generally required to wear numbers within ranges based on their positions as shown in the following table. Exceptions to this system do exist, including during the National Football League preseason with associated larger team rosters. The numbers used relate to the player's primary position when they are first assigned a number. If they later change positions, they can keep their prior number, unless it conflicts with the eligible receiver rule; that is only players that change positions from an eligible position (such as receiver or back) to an ineligible position (such as an offensive lineman) are required to change numbers if they change position. Additionally, during a game a player may play out-of-position, but only after reporting in to the referees, who will announce to the stadium that a specific player number has reported in (for example "Number 61 has reported as an eligible receiver") to alert the opposing team, other officials, and the audience that a player is legally out-of-position. Although the NFL does allow teams to retire jersey numbers, the league officially discourages the