Pertinent info in bold at the end.
Main article: Squad number
In the NFL, players wear uniform numbers based on the position they play. The current system was instituted into the league on April 5, 1973, as a means for fans and officials (referees, linesmen) to more easily identify players on the field by their position. Players who were already in the league at that date were grandfathered, and did not have to change their uniform numbers if they did not conform. Since that date, players are invariably assigned numbers within the following ranges, based on their primary position:
Quarterbacks, placekickers and punters: 1–19
Wide Receivers: 10-19 and 80-89
Running backs and defensive backs: 20–49
Offensive linemen: 50–79
Linebackers: 50–59 and 90–99, or 40-49 if all are taken
Defensive linemen: 60–79 and 90–99
Tight ends: 80–89, or 40–49 if all are taken
Prior to 2004, wide receivers were allowed to only wear numbers 80–89. The NFL changed the rule that year to allow wide receivers to wear numbers 10–19 to allow for the increased number of players at wide receiver and tight end coming into the league. Linebackers are allowed to wear numbers between 40-49 when all of 50-59 and 90-99 numbers are taken. Prior to that, players were only allowed to wear non-standard numbers if their team had run out of numbers within the prescribed number range. Keyshawn Johnson began wearing number 19 in 1996 because the New York Jets had run out of numbers in the 80s. Oakland Raider offensive center Jim Otto wore a 00 jersey during most of his career with the AFL team and kept the number after the leagues merged.
Occasionally, players will petition the NFL to allow them to wear a number that is not in line with the numbering system. Brad Van Pelt, a linebacker who entered the NFL in 1973 with the New York Giants, wore number 10 during his 11 seasons with the club, despite not being covered by the grandfather clause. In 2006, New Orleans Saints running back Reggie Bush petitioned the NFL to let him keep the number 5 which he used at USC. His request was later denied. Former Seattle Seahawks standout Brian Bosworth attempted such a petition in 1987 (to wear his collegiate number of 44 at the linebacker position), also without success. The Seahawks attempted to get around the rule by listing Bosworth as a safety, but after he wore number 44 for a game against the Kansas City Chiefs, the NFL ruled Bosworth would have to switch back to his original number, 55.
It should be noted that this NFL numbering system is based on a player's primary position. Any player wearing any number may play at any position on the field at any time (though offensive players wearing numbers 50–79 and wishing to play at end or back must let the referee know that they are playing out of position by reporting as an "ineligible number in an eligible position"). Normally, only players on offense with eligible numbers are permitted to touch the ball by taking a snap from center, receiving a hand-off or catching a pass. It is not uncommon for running backs to line up at wide receiver on certain plays, or to have a large lineman play at fullback or tight end in short yardage situations. Also, in preseason games, when teams have expanded rosters, players may wear numbers that are outside of the above rules. When the final 53-player roster is established, they are reissued numbers within the above guidelines.