The numbering system used today originated when all teams employed some variation of the single wing formation on offense.
Examples are Otto Graham going from 60 to 14, Norm Van Brocklin going from 25 to 11, Marion Motley going from 76 to 36, and Tom Fears going from 55 to 80.
[citation needed] The American Football League (AFL) of the 1960s, which also later merged with the NFL, used essentially the same numbering system as the NFL with some exceptions, mostly pertaining to wide receivers, who were allowed to wear numbers in the teens and 20s (as the AFL had a greater priority toward offense, the league often made use of flankers, receivers positioned in the backfield).
When it went into effect, players who had played in the league before the 1973 season were given a grandfather clause to continue wearing their now-prohibited numbers.
Pro Football Hall of Fame wide receiver Charlie Joiner was the last player covered by the clause, wearing number 18 through the 1986 NFL season, after which he retired (Under the current jersey system, Joiner's #18 would have been allowed for a wide receiver to wear).
[7] Devin Hester is another example; he was originally drafted as a cornerback and was allowed to keep his number 23 when he converted to a wide receiver.
The rulebook also allows players to appeal for exemptions to the numbering rules directly to the commissioner's office, which may grant such exceptions on occasion.
Linebacker Bryan Cox wore 0 in the 2001 preseason with the New England Patriots; for the regular season, he switched to 51.
[8] In addition to Jim Otto, wide receiver Ken Burrough of the Houston Oilers and halfback Steve Bagarus of the Washington Redskins and Los Angeles Rams also wore the number 00 during their NFL careers.
Starting with the 2023 season, the number 0 was once again allowed to be issued to any player except offensive and defensive linemen.
[9] Notable players who switched to number 0 include running back D'Andre Swift of the Philadelphia Eagles, wide receiver Calvin Ridley of the Jacksonville Jaguars, and linebacker Roquan Smith of the Baltimore Ravens.