Key

1929–1962

Minor League Baseball operated with five classes (Double-A, Class A, Class B, Class C, and Class D) from 1929 to 1935.^[2] Class A1, between Double-A and Class A, was added in 1936.^[3] The minors continued to operate with these six levels through 1945.^[3] Triple-A was established as the highest classification in 1946, and Class A1 became Double-A, with Class A through D remaining.^[3] These six levels continued through 1962.^[2] The Pacific Coast League (PCL) was reclassified from Triple-A to Open in 1952 due to the possibility of becoming a third major league. This arrangement ended following the 1957 season when the relocation of the National League's Dodgers and Giants to the West Coast ended any chance of the PCL being promoted.^[2]

1963–1989

Prior to the 1963 season, Major League Baseball (MLB) initiated a reorganization of Minor League Baseball that resulted in a reduction from six classes to four (Triple-A, Double-A, Class A, and Rookie) in response to the general decline of the minors throughout the 1950s and early-1960s when leagues and teams folded due to shrinking attendance caused by baseball fans' preference for staying at home to watch MLB games on television. The only change made within the next 27 years was Class A being subdivided for the first time to form Class A Short Season in 1966.^[2]

1990–2020

Minor League Baseball operated with six classes from 1990 to 2020. In 1990, the Class A level was subdivided for a second time with the creation of Class A-Advanced. The Rookie level consisted of domestic and foreign circuits.^[2]

2021–present

The current structure of Minor League Baseball is the result of an overall contraction of the system beginning with the 2021 season. Class A was reduced to two levels: High-A and Low-A.^[100] Low-A was reclassified as Single-A in 2022.^[101]