Throughout the history of baseball, the rules have frequently changed as the game continues to evolve. A few common rules most professional leagues have in common is that four balls is a base on balls, three strikes is a strikeout, and three outs end a half-inning.

Baseball is a bat-and-ball sport played between two teams of nine players each, taking turns batting and fielding. The game occurs over the course of several plays, with each play generally beginning when a player on the fielding team, called the pitcher, throws a ball that a player on the batting team, called the batter, tries to hit with a bat. The objective of the offensive team is to hit the ball into the field of play, away from the other team's players, allowing its players to run the bases, having them advance counter-clockwise around four bases to score what are called "runs". The objective of the defensive team is to prevent batters from becoming runners, and to prevent runners' advance around the bases. A run is scored when a runner legally advances around the bases in order and touches home plate.