When a game is tied after regulation play, sometimes seven innings in the minor leagues, but always nine innings in the major leagues, the game will go into extra innings.
In extra innings, the game continues as usual until the away team has scored more runs and the home team has completed a turn at bat, or the home team has scored more runs thus ending any chance for the away team to come back.
Before the advent of lights, extra inning games were often terminated and called a tie due to darkness. When lights were added to Wrigley Field in 1988, ties due to darkness were eliminated in the major leagues.
The most extra innings played in professional baseball is 24 by Rochester at Pawtucket in the International League on April 18, 1981. The record in the major leagues is 18 on May 1, 1920 between Brooklyn and Boston at Braves Field. The game was called a tie due to darkness.
In Nippon Pro Baseball, teams play three extra innings. If a team has not won after twelve innings the game is declared a tie. The exception is in the Japan Series and playoffs, where the game is played until it is decided conclusively. In the Korea Baseball Organization, the limit is also 3 extra innings, even in the Korean Series, resulting in occasional ties in the most important matches of the year.
In both the spring and summer Koshien Tournaments, games will last a maximum of 15 innings. If the score is still tied, the game will end in a tie and be replayed the next day.
The "Schiller Rule"
In the 2008 World Junior Championship and 2008 Olympics, the IBAF introduced a new twist for the play of extra innings for that competition. If the game is still tied after the 10th inning, both teams then start the 11th inning with two runners on base - occupying first and second base. The manager is free to chose any players in the line-up as his runners, as long as they bat consecutively. The player following the two runners in the batting order then leads off the inning at the plate. If a 12th inning is required, the batter whose turn is due comes to the plate to lead off as scheduled, and the two batters that precede him in the line-up start the inning on base, the batter before him on first base, and the batter before that on 2nd. For example, if the 4th hitter was the last to bat in the 11th inning, the 12th inning will start with the 3rd hitter on second base, the 4th hitter on first base, and the 5th hitter at the plate. The objective is to encourage the scoring of runs in extra innings in order to bring the game to a quick conclusion. This controversial rule has since been adopted in the Hoofdklasse and Cuban Serie Nacional, among other top level leagues. The rule was dubbed the Schiller Rule by American writer Peter Bjarkman in honor of former IBAF president Harvey Schiller, and the name has become commonly used in Cuba.
The rule was also used in the 2017 World Baseball Classic, from the 11th inning on. In this case, the manager was not allowed to decide who would bat first in the 11th: it was up to the next scheduled hitter in the batting order to go first, with the two batters preceding him taking their place on first and second base.
In 2017, Organized Baseball decided to test a similar rule in two low-level leagues, the Gulf Coast League and Arizona League; under this rule, each inning after the 9th started with the last runner to have completed a plate appearance in the 9th inning already on second base. The official explanation for the introduction of the rule was to test its practical implications and assess whether it could be adopted as a measure to speed up the game. In 2018, the rule was proposed for inclusion in the All-Star Game, starting in the 11th inning, and in spring training games in the 10th inning (spring training games were limited to just one extra inning). In both cases, the variation used was to start the inning with a runner on second base only. However, the change in spring training did not go through because of opposition from the Players' Association. But the rule was extended to the entire minor league structure of organized baseball that season that year, starting in the 10th inning, in order to avoid overlong extra-inning games; it had already been in place in certain independent leagues.
It has been speculated that the Schiller Rule gives the advantage to the visiting team, as the visiting manager can keep his closer for an eventual bottom of the inning after he takes the lead, an option not open to the home team manager. In practice, with the rule variation featuring two baserunners, the first batter called to the plate in an inning will almost always attempt a sacrifice bunt, with everyone in the ballpark aware of the strategy. If it is successful, it is almost guaranteed that the defense will issue an intentional walk to the next batter in order to load the bases. The true strategy only starts with the following batter, as the defensive team needs to decide whether they will play their infield in in order to cut off the potential winning run at the plate, or play further back in the hope of a possible inning-ending double play. The team at bat also has the option of laying down a risky squeeze bunt at that point. One other point to note is that any run scored by a runner already on base at the start of the inning is considered to be unearned, no matter how it is scored.
- Philip J. Lowry: Baseball's Longest Games: A Comprehensive Worldwide Record Book, McFarland, Jefferson, NC, 2010.
- Bob Nightengale: "Weird 11th-inning tiebreaker works for World Baseball Classic - but never for MLB", USA Today Sports, March 12, 2017. 
- Jorge L. Ortiz: "In testing drastic new extra-innings rule, MLB grapples with pace, tradition", USA Today Sports, February 9, 2017. 
- Joe Posnanski: "Extra innings can be too much of a good thing: Low Minors experimenting with beginning extra frames with runner on second", mlb.com, February 9, 2017.