Baseball is the only major team sport with no clock, which means a tied game can, in principle, last forever. Extra innings are how it ends, and the way they are played has changed significantly.
The basic structure
If the score is tied after nine innings, the teams play a tenth. Each side gets its half of the inning. If the home team is ahead after the visiting team bats in the top of the inning, the game ends immediately and the home side does not need to bat. If the game is still level after both halves, they play another inning. There is no limit.
The automatic runner
In extra innings, each team begins its half-inning with a runner already standing on second base. That runner is the player who made the final out of the previous inning, or a pinch-runner for them.
The runner is not charged to the pitcher as an earned run if they score. The scoring rules treat the situation as if the runner reached on an error, which protects pitchers from a circumstance they did not create.
The purpose is blunt: to end games faster. A runner on second with nobody out converts a sizeable share of innings into a run, so the tenth or eleventh inning usually settles it rather than the sixteenth.
Where it does not apply
The automatic runner is a regular-season rule. Postseason extra innings are played conventionally, bases empty, and the game goes as long as it goes. The reasoning is that a championship should not be decided by a runner nobody put on base.
The strategy it created
The visiting team, batting first, has to decide between playing for one run and playing for a big inning. A sacrifice bunt moves the runner to third with one out, making a sacrifice fly enough, but a single run is often not enough, because the home team will start its own half with a runner on second too.
The home team has a real advantage. It knows exactly how many runs it needs, and it can play for one run at a time with the tying and winning runs already in scoring position.
Why people argue about it
Traditionalists see a run that nobody earned. Marathon games were part of the sport's identity, and the longest games are treasured oddities.
The counter-argument is practical. Extra-inning marathons destroyed bullpens for a week, forced position players to pitch, and sent fans home at two in the morning. Shortening them protects players across a 162-game season.
The short version
Tied after nine, play another inning. In the regular season each half-inning starts with a runner on second and that run is unearned. In the postseason, no free runner, just baseball until somebody wins.