Innings and Determining a Winner

An inning consists of each team having one turn in the field and one turn to hit, with the visiting team batting before the home team. A standard game lasts nine innings, although some leagues (such as minor leagues and high school baseball) use seven-inning games. The team with the most runs at the end of the game wins. If the home team is ahead after eight-and-a-half innings have been played, it is declared the winner, and the last half-inning is not played. If the home team is trailing or tied in the last inning and they score to take the lead, the game ends as soon as the winning run touches home plate; however, if the last batter hits a home run to win the game, they and any runners on base are all permitted to score.

If both teams have scored the same number of runs at the end of a regular-length game, a tie is avoided by the addition of extra innings. As many innings as necessary are played until one team has the lead at the end of an inning. Thus, the home team always has a chance to respond if the visiting team scores in the top half of the inning; this gives the home team a small tactical advantage. In theory, a baseball game could go on forever; in practice, however, they eventually end. In Major League Baseball the longest game played was a 26-inning affair between the Brooklyn Robins and Boston Braves on May 1, 1920. The game ended in a 1-1 tie called on account of darkness.

In Major League Baseball, games end with tie scores only because conditions have made it impossible to continue play. A tie game does not count as an official game in the standings unless it is finished later or replayed; however, individual player statistics from tie games are counted. Inclement weather may also shorten games, but at least five innings must be played for the game to be considered official; four-and-a-half innings are enough if the home team is ahead. Previously, curfews and the absence of adequate lighting caused more ties and shortened games.

In Japanese baseball, if the score remains tied after nine innings, up to three extra innings may be played before the game is called a tie. Some youth or amateur leagues will end a game early if one team is ahead by ten or more runs, a practice known as the "mercy rule" or "slaughter rule". Rarely, a game can also be won or lost by forfeit.

There is a short break between each half-inning during which the new defensive team takes the field and the pitcher warms up. Traditionally, the break between the top half and the bottom half of the seventh inning is known as the seventh-inning stretch. During the "stretch," fans often sing the chorus of "Take Me Out to the Ball Game."

Substitutions

Each team is allowed to substitute for any player at any time, but no player, once removed from the game, may return. A batter who replaces another batter is referred to as a pinch hitter; similarly, a pinch runner may be used as a replacement for a baserunner. Any replacement is a permanent substitution; the replaced player may not return to the game.

It is common for a pitcher to pitch for several innings and then be removed in favour of a relief pitcher. Because pitching is a specialised skill, most pitchers are relatively poor hitters; it is common to substitute for a pitcher when they are due to bat. This pinch hitter is typically then replaced by a relief pitcher when the team returns to the field on defence, but more complicated substitutions are possible, most notably the double switch.

Many amateur leagues allow a starting player who was removed to return to the game in the same position in the batting order under a re-entry rule. Youth leagues often allow free and open substitution to encourage player participation.

Most leagues, notably Major League Baseball's American League, allow a designated hitter, a player whose sole purpose is to hit when it would normally be the pitcher's turn. This is not considered a substitution but rather a position, albeit a purely offensive one. A designated hitter does not play in the field on defence and may remain in the game regardless of changes in pitchers.

Rosters

During the course of a game, each baseball team has players that are an active part of the game, called "in the game," and players that are not, called "on the bench." The players on the bench are needed in case of injuries and to make strategic pitching, fielding, and batting substitutions. To keep the game fair, each team is limited to a fixed number of players. That number is dictated by the rules of the game.