Uniform numbers in American football are unusual compared to those in other sports. They are displayed in more locations on the uniform; they are universally worn on both the front and back of the jersey; and in many cases "TV numbers" are displayed on either the jersey sleeves, the shoulder pad, or occasionally on the helmets. The numbers on the front and back of the jersey also are very large, covering most of the jersey. More importantly, certain numbers may only be worn by players playing specific positions; thus, the jersey numbers assist the officials in determining possible rules infractions by players. Under current rules in all three of the most prominent levels of American football, all players must wear a number from 1 to 99, and no two players on the same team may wear the same number on the field at the same time. Players could formerly use the numbers 0 and 00, numbers that were phased out in the 1970s, and on two special occasions in the 1960s, placekickers wore the number 100. Use of the number 0 will again be allowed by the NCAA in American college football, starting with the 2020 season. 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. The related sport of Canadian football follows a similar numbering scheme to that of American scholastic football, except that the ineligible numbers span only from 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 2020, 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 to an ineligible position 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 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 practice for fear of teams running out of numbers; the rule book requires teams to make available retired numbers for new players should they exhaust all available numbers at a particular position. In the XFL, the NFL numbering conventions are followed with a slight exception being that wide-receivers are allowed to wear single-digit numbers.
NCAA
According to NCAA rule book, Rule 1 Section 4 Article 1 "strongly recommends" numbering as follows for offensive players:
Otherwise all players can be numbered 1–99; the NCAA makes no stipulation on defensive players. Two players may also share the same number, although they may not play during the same down. Starting in the 2020 NCAA football season, the use of duplicate numbers will be restricted to only two players on a team, and players will be allowed to wear No. 0. Two players, both placekickers, have worn No. 100 in NCAA history. Both did so as part of centennial celebrations in the 1960s, with special dispensation from the NCAA: West Virginia's Chuck Kinder, who did so in 1963 to commemorate the state of West Virginia's 100th anniversary, and Kansas'Bill Bell, who did so in 1969 to commemorate 100 years since the first organized college football contest.
High school
On high school and other lower youth teams, jerseys with different number ranges are different sizes, and since many of these teams do not reorder jerseys every year, players are often assigned numbers based more on jerseys that fit them rather than specific position. Odessa Permian High School plays in Texas, where NCAA rules are used; yet Permian's tradition is that quarterbacks will wear numbers in the 20s unlike most schools in college or high school. Although previous editions of the National Federation of State High School Associations rule book indicated a recommended numbering system nearly identical to the NCAA's, later editions from approximately 2000 onward only indicate the bare minimum requirements: offensive linemen must be numbered from 50 to 79, while all other positions including backs and ends must wear numbers either from 1 to 49 or 80 to 99.
