Fatto generale: 21 is the number of points required to win a game in badminton and table tennis (before 2001).
Fatto matematico: 21 is the smallest number of differently sized squares needed to square the square.
Trivia: 21 is the number of points required to win a game in badminton and table tennis (before 2001).
Data storica: January 21st is the day in 1908 that New York City passes the Sullivan Ordinance, making it illegal for women to smoke in public, only to have the measure vetoed by the mayor.