They are organized by one of five classes (from highest to lowest): Triple-A, Double-A, High-A, Single-A, and Rookie.
Additionally, three leagues consisting of a total of 85 Rookie teams are located at MLB spring training complexes in Arizona and Florida, as well as in the Dominican Republic.
The Arizona Fall League, consisting of six teams, operates in the autumn after the conclusion of the MiLB and MLB seasons to develop top prospects at various classifications.
[3] The Double-A classification is the second-highest level of play in Minor League Baseball.
[3] The Rookie classification is the lowest level of play in Minor League Baseball.
[12] The independent, single-site United Shore Professional Baseball League consists of four teams.