I'll give a brief history of classifications as well...
In 1902, the National Association of Professional Baseball Leagues
was formed and existed with the following classifications from highest
to lowest:
A, B, C, D
In 1912, the classification of AA was established as the highest classification,
the system was as follows:
AA, A, B, C, D
In 1936, the classification of A1 was established which was between
AA and A, creating:
AA, A1, A, B, C, D
In 1938, the classification of E was established to be the lowest classification
(the only Class E League - Twin Ports League (1943)):
AA, A1, A, B, C, D, E
In 1946, the system was restructured with AA leagues becoming AAA and
A1 leagues becoming AA:
AAA, AA, A, B, C, D, E
In 1952, the classification of Open was established for the Pacific
Coast League to help it become a major league:
Open, AAA, AA, A, B, C, D, E
In 1958, the classification of Open was removed when the National League
moved into Los Angeles and San Francisco:
AAA, AA, A, B, C, D, E
In 1963, the system was radically restructured to the system in its
current form, as listed above.