Why do all NFL Football teams have #21 on the back of their helmets?

Every single team has the number 21 somewhere on the back of their helmets. Why?

Why do all NFL Football teams have #21 on the back of their helmets?nba live 2007

dead player***

Why do all NFL Football teams have #21 on the back of their helmets?nba betting ,nba teams

because of the dead redskin

shaun taylor (#21)
In memory of Redskins Sean Taylor
In memory of Sean Taylor (1983-2007)
The week following the killing of Sean Taylor, all the teams in the NFL wore the number 21... since then, some have stopped wearing the number...

The New England Patriots, wear the number 91 on the back of their helmets, and have since the first game of the 2007 season, in memory of their teammate Marquise Hill, who was killed trying to save a friend, in a jet ski accident, last summer.
Sean Taylor
for Sean Taylor
Redskins' Sean Taylor.
The NFL did this in memory of Washington Redskin Sean Taylor who was killed in his home last month.
because thats jordans number???
It's because of Sean Taylor and NO, it WASN'T on before his death. You must've been seing things buddy. You may have been seen the number 91 though.
In memories of Sean Taylor #21 of Washington Redskins who died recently after being shooted
In memory of Sean Taylor, who was killed in his home while holding a machete.
For the Washington player Sean Taylor who died.
that was sean taylors number
sean taylor, the football player who was killed when someone was robbing his house
in memory of sean taylor, that football player who was killed
in honor of #21 Sean Taylor of the Washington Redskins who was killed recently.
it was shawn taylor's number.
'Cause they can't wait to get to Las Vegas to play their favorite game "21"
I think its b/c of kevin everett, who was seriously injured earlier this year