The first team to play baseball under modern rules were the New York Knickerbockers.
Clubs now had the ability to enforce player contracts, preventing players from jumping to higher-paying clubs.
Clubs in turn were required to play their full schedule of games, rather than forfeiting games scheduled once out of the running for the league championship, as happened frequently under the National Association.
A concerted effort was made to reduce the amount of gambling on games which was leaving the validity of results in doubt.
At the same time, a "gentlemen's agreement" was struck between the clubs which endeavored to bar non-white players from professional baseball, a bar which was in existence until 1947.
Aided by the Civil War, membership grew to almost 100 clubs by 1865 and to over 400 by 1867, including clubs from as far away as California.