Classification System

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The Minor League Classification System organises minor league baseball leagues into different levels.

Following the reorganization of the minor leagues into the Professional Development League in 2021, there are now five levels: AAA, AA, High-A, Low-A and complex-based leagues (replacing the former Rookie League classification).


Before 1902, there was a classification system in place in minor league baseball based on team salary limits.

In 1902, the National Association of Professional Baseball Leagues was formed and its leagues were divided into four classes; A, B, C, D. The classes were based the total population of all of the league's member cities. The initial breakdowns were:

  • A: 1,000,000 and over
  • B: 400,000 to 999,999
  • C: 200,000 to 399,999
  • D: up to 200,000

In 1912, the AA was established as the highest classification. The A1 class was established in 1936 between the AA and A. Class E was established in 1938 as the lowest classification, however there was only class E league, the Twin Ports League in 1943.

In 1946, the classification system was restructured; with AA leagues become AAA and A1 leagues becoming AA and class E was dropped. In 1952, the classification of Open classification was established to help the Pacific Coast League become a major league. It was the top classification. In 1958, the Open classification was removed after the National League moved into Los Angeles and San Francisco.

In 1963, the system was radically restructured into four classifications; AAA, AA, A, Rookie. Classification was no longer based on population, but instead assigned. This is considered the start of the modern era of the minor leagues. The Northern League (A) began to play a short-season schedule in 1965, creating a de facto A-Short Season classification.

Some time around 1992, the A and Rookie classes were informally split into two levels, advanced and regular, creating the seven-tiered system which lasted until 2019.

Further Reading[edit]

  • John Cronin: "Truth in the Minor League Class Structure: The Case for the Reclassification of the Minors", in The Baseball Research Journal, SABR, Volume 42, Number 1 (Spring 2013). [1]
  • Jonathan Mayo: "MLB announces new Minors teams, leagues: New model includes player salary increases, modernized facility standards, reduced travel", February 12, 2021. [2]

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