From BR Bullpen
Uniform numbers are on backs, and sometimes both, fronts and backs of uniforms of baseball players, coaches, managers, and most baseball team personal. Numbers are commonly used to identify one player from another.
One of the highest honors a team can pay to one of its players is to have his uniform number retired, i.e. not used by any other player after him. One uniform number, 42, is retired throughout major league baseball in honor of Jackie Robinson, the pioneer of baseball's integration. That number is worn by all players once a year, on "Jackie Robinson Day", however, in tribute to Robinson. There is a movement asking to have similar treatment reserved for number 21 worn by Hispanic pioneer Roberto Clemente, but Major League Baseball has not acquiesced.