As football developed, players at certain positions began to migrate to certain numbers, and the N.F.L. began to codify its numbering rules in the 1950s. The big change came in 1973, when strict rules were put in place, tying numbers to position.

In other sports, numbers are primarily used so that fans can identify players, and perhaps to allow a coach to bellow, “Stop No. 34!” So restrictions are few. At many levels of basketball, but not in the N.B.A., players are limited to numbers with digits from 0 to 5. That way referees can signal who fouled using just the fingers of one hand.

But nobody is going to mind too much if No. 27 is worn by a slugging center fielder, Mike Trout, on the Angels, and an ace starting pitcher, Aaron Nola, on the Phillies.

In football, though, numbers help officials tell if players are improperly lined up out of position. Do you see No. 66 (reserved for linemen) lined up in the backfield or out wide? There’s probably an illegal formation flag on the way.

At least one star player is not a fan of the new rules. “Good luck trying to block the right people now,” wrote Tom Brady on Instagram. “Why not let the linemen wear whatever they want too? Why have numbers? Just have colored jerseys.”

“Dumb,” he added.

Numbers matter to players. Every season, stories appear about rookies or traded players seeking to grab a desired number from a teammate who already wears it. Sometimes the dispute is resolved with payments that have on occasion reached five figures. The N.F.L.’s new rules may mean some pro players will be able to return to numbers they wore and liked in college, which has more permissive rules.

Still missing from the N.F.L. are 0 and 00, currently worn by more than 30 N.B.A. players, including Carmelo Anthony (0) and Damian Lillard (00). Those numbers have been banned from the N.F.L. since the 1973 reforms, although the Hall of Fame center Jim Otto wore 00 in the ’60s and ’70s.