The Three Batter Minimum
A reliever must face three hitters or finish the inning, which ended a whole job.
What it actually is. Since 2020 a pitcher entering a game must face at least three batters or pitch until the inning ends, unless he is injured. The rule was introduced to reduce the number of mid inning pitching changes and shorten games.
What somebody who knows the game gets out of it. The rule eliminated the left handed one out specializt as a roster role, which was the point, and it changed bullpen construction more than game length. Clubs now need relievers who can retire hitters from both sides, so the value of a genuine platoon split arm fell and the value of a pitch that works against opposite handed hitters, usually a changeup or a cutter, rose. A rule about pace reshaped what kind of pitcher gets developed.
The numbers worth knowing. Three batters, or the end of the inning. Game length fell by only a few minutes, far less than the pitch clock later achieved.
What people get wrong about it. Finishing the inning satisfies the rule, so a reliever who comes in with two out and gets one hitter has met it. That is why you still see one batter appearances and why they are not violations.
At Little League and travel ball. No equivalent, and youth pitching changes are governed by pitch counts and by rules about a pitcher who has been removed from the mound returning to it, which vary by sanction and are covered in the travel rulebook section.
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