Retired Uniform Numbers in the American League
Uniform numbers were used on a permanent basis by the Yankees and Indians in 1929, so that fans could tell who was who on the field. Those uniform numbers corresponded to the player's place in the lineup; those who batted leadoff wore number one, those who batted clean-up wore number four and so on.
This practice caught on and within a few years every team wore numbers. Shortly after it caught on, it began to change as numbers were retired by teams making the lineups impossible to complete thereby allowing players to wear other numbers not corresponding to the position in the lineup.
The following page is a comprehensive breakdown of every retired uniform number in the American League.