They played for “health and recreation merely,” but they also showed a lively interest in improving the game. Cartwright, along with the Knickerbockers’ president, a physician from New Hampshire named Daniel Lucius “Doc” Adams, would help draw up a set of new rules that changed baseball forever.
The Knickerbockers decreed that the infield be diamond-shaped, rather than square. First and third bases were set forty-two paces apart. The balk was identified – and outlawed. Foul lines were established. Pitchers were to throw the ball underhand, keeping the elbow and wrist straight. The batter got three missed swings before he was called out. More important, runners were to be tagged or thrown out, not thrown at.

Baseball: An Illustrated History, Geoffrey C. Ward and Ken Burns

Because Adams’s harder, more resilient ball could travel farther faster than its predecessor, the need for relaying was greatly reduced, and the shortstop – Doc Adams himself pioneered the position – for the first time moved to the infield.

Baseball: An Illustrated History, Geoffrey C. Ward and Ken Burns

To keep their game unsullied, the Knickerbockers and fifteen other clubs that now played by their rules banded together in 1857 to form the National Association of Base Ball Players, with Adams as its president. Its goal was to “promote additional interest in baseball playing” and “regulate various matters necessary to [its] good government and continued respectability.”

Baseball: An Illustrated History, Geoffrey C. Ward and Ken Burns

Doc Adams left the Knickerbockers in 1862, he remembered, “but not before thousands were present to witness matches and any number of outside players standing ready to take a hand on regular playing days… We pioneers never expected to see the game so universal as it has … become.”

Baseball: An Illustrated History, Geoffrey C. Ward and Ken Burns