The term the 12th man has been used by football teams regarding their fans. It has been used for quite some time, however, the Seattle Seahawks have made the term quite popular since the team has become really good in the last three years.
The Seahawks made it important in 1984 by retiring the number so there are no players on Seahawks using the number. Right now the team is winning at a high level, and actually last year winning the Super Bowl for the first time. Also, Century Link Field the home for the Seahawks has the best home field advantage, and the fans are known for being very loud so much that often the opponent will get be charged a penalty for taking too long to run a play.
Another thing fans can purchase a number 12 jersey saying fan on it. Also, they have a 12th man flag that is always being raised before any home game as well being seen on the space needle. Most of the time a former Seahawk player will raise the flag, however, a known celebrity sometimes will. I see people that have a 12th man flag hanging outside their home or on their car as well.
Last year during the Super Bowl season Seattle Seahawks coach Pete Carroll decided to start what is called Blue Friday. What that means that every 12th man who has a Seahawk jersey wears it to work. One thing for sure I see many Seahawk jerseys on Friday as well as on Sunday. You see a lot of men, women, and children wearing Seahawk jerseys. Some I see them wearing them I would never expect to be Seahawk fans without the jersey.
The Seahawk jersey not only has become big in Washington state but around the whole northwest. If you see someone wearing a Seahawk jersey say 12th man.
It is so amazing no matter where you go, or what sport you are talking about there are crazy fans who love their teams.