The sides of a hendecagon (sometimes also known as an undecagon or unidecagon), the players on an association football (soccer) team and the players on a cricket team all come in sets of 11. Likewise, at least according to some theoretical physicists, the dimensions of space time also number 11.
The hendecagon is unusual among regular polygons, since it cannot be constructed by way of the classical Greek rules of geometry. The shape was used for the inside border of the rim of the Susan B. Anthony United States Dollar.
Association football (or soccer) teams have had eleven players since the standardization of the game in 1870. Prior to this time, a medieval European precursor to modern association football had teams of varying numbers — usually anywhere between 15 and 21. The number of players on each team was also not necessarily equal. Sometimes, as in the case of Calcio Fiorentino, which was an early ancestor of football, there would be as many as 27 players on a team, five of which were goalkeepers.
It is believed by some that the decision to limit teams to 11 players specifically was taken in a bid to increase this new sport's popularity among people who were, at the time, more used to cricket. Since cricket teams also have 11 players, copying this aspect of the game may have made football easier to follow.