FL: 1913-1915 (3 seasons)
Eliminated the 'reserve clause', allowing free agency that created competition and disdain with the AL and NL.
USBL: 1912-1912 (1 season)
Sputtering through two short seasons, the league was the first to introduce free agency and multi-year contracts.
WL: 1894-1900 (7 seasons)
Originally a minor league in 1885, evolved into a major league that eventually became the modern American League.
PL: 1890-1890 (1 season)
Known as "The Brotherhood" league, it was formed around players from the National League's first players' union.
UA: 1884-1884 (1 season)
The league president's favoritism toward his own St. Louis franchise doomed the league from the beginning.
AA: 1882-1891 (10 seasons)
Became a significant force in baseball by playing on Sundays and allowing sales of alcohol, unlike the NL.
NL: 1876- (144 seasons)
The "Senior Circuit" is the world's oldest professional team sports league still in existence today.
NA: 1871-1875 (5 seasons)
The league included most of the pro clubs and highest caliber players of its time, paving the way for the origin of the NL in 1876.
NABBP: 1857-1870 (14 seasons)
The first organization governing professional baseball and establishing standard rules across the national sport.