Uniform number (Major League Baseball)

Uniform numbers are used to tell and identify different baseball players's and coach's. Over time, numbers can have a much more meaning to the player and fans. A number can be symbolic of a player's legacy, and has resulted in all kinds of superstition (or rumors). Uniforms numbers are put on the backs of baseball uniforms.
Retired numbers in Major League Baseball.
Many teams retire numbers of important players who used to play for their team. Retiring a number means that nobody on that team can use that number anymore. Players' numbers are not retired while they are still playing baseball.
The number "42" was retired by all of Major League Baseball in 1997 because it was used by Jackie Robinson, who was the first black player in Major League Baseball. Players who were already wearing #42 were allowed to keep using the number, but nobody else could use it. As of 2010, New York Yankees pitcher Mariano Rivera was the last player using #42.