Under the standard rules of American football, players in the National Football League wear uniform numbers between 1 and 99, and no two players may wear the same number on the field at the same time. Additionally, those wearing 50 to 79 are prohibited from catching or touching forward passes if their team is in possession of the ball, unless explicitly indicated to the referee during a Tackle-eligible play. In addition to these standard stipulations, the NFL requires players numbers to be within a certain range according to their primary position, although exceptions exist. Numbers can be retired, but the practice is discouraged by the NFL.