Topic summary
Uniform number

Extracted from the Wikipedia article Number (sports).
Baseball
In the early years of baseball, teams did not wear uniform numbers. Teams experimented with uniform numbers during the first two decades of the 20th century, with the first Major League team to use them being the 1916 Cleveland Indians which used them on their left sleeves for a few weeks before abandoning the experiment. Again in 1923, the St. Louis Cardinals tried out uniforms with small numbers on the sleeves, but the players did not like them, and they were removed. For the 1929 Major League Baseball season both the New York Yankees and Cleveland Indians put numbers on their jerseys, the first two teams to do so, beginning a trend that was completed by 1937, when the Philadelphia Athletics became the last team to permanently add numbers to their jerseys.