Players League
The Players League was an attempt to establish a third major baseball league in 1890. The National League and the American Association already existed as major leagues. The PL lasted just the one season of 1890, and the Boston franchise won the championship that year.
Players League franchises
Boston and Philadelphia subsequently joined the American Association when the Players League folded. Brooklyn merged with the Brooklyn Bridegrooms (later the Brooklyn Dodgers) of the National League after the season.