Baseball Rules: Everything That Changed, and When
TL;DR
The core of baseball is unchanged, but the game around it has been redesigned in four seasons. A pitch clock arrived in 2023 at 15 seconds with the bases empty and 20 with runners on, cut to 18 in 2024. Defensive shifts were restricted, bases grew from 15 to 18 inches, and from the 2026 season players can challenge ball and strike calls to an automated system. Most baseball rules pages online still describe the game as it was in 2022.
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Four seasons that rewrote the game
Baseball spent decades with a reputation for refusing to change anything. Then it changed almost everything at once.
| Season | Change |
|---|---|
| 2023 | Pitch clock introduced — 15s bases empty, 20s runners on |
| 2023 | Defensive shift restrictions |
| 2023 | Bases enlarged from 15in to 18in |
| 2024 | Pitch clock with runners on cut from 20s to 18s |
| 2026 | ABS — automated ball-strike challenges |
If a page describing baseball’s rules does not mention a clock, it predates 2023. If it says 20 seconds, it predates 2024. If it does not mention challenges, it predates this season. That is a faster decay rate than almost any other major sport’s rulebook.
The pitch clock
The clock is the change that altered how the sport actually feels.
15 seconds with the bases empty. 18 seconds with runners on. The pitcher must have begun their motion before it expires.
The penalty is automatic and applied without discretion. Pitcher too slow: an automatic ball. Batter not ready in time: an automatic strike. No warning, no umpire judgement about whether the delay was reasonable.
Pitchers also get a limited escape. They may step off and reset the clock twice per plate appearance without penalty — the disengagement limit, which quietly doubles as a restriction on pickoff attempts, since each one uses a disengagement.
The effect was immediate and large: average game time fell by roughly 24 minutes in the first season compared with 2022. Two seconds were then taken back off in 2024, which is a useful signal about how the committee views the change — not a one-off correction but an ongoing dial.
Basketball reached the same conclusion about dead time much earlier, and its family of clocks does the same job.
The shift restriction
Widely described as a shift ban. It is narrower than that.
When the pitch is released, the defence must have two fielders on each side of second base, and all four infielders must have both feet on the infield dirt.
What that outlaws is the extreme alignment — three infielders stacked on one side against a pull hitter, or a fielder standing in shallow outfield grass. What it still permits is ordinary positioning: fielders may shade heavily toward a hitter’s tendencies, so long as they stay on their own side of the bag and on the dirt.
So the rule does not require symmetrical defence. It requires the defence to remain recognisably an infield.
Bigger bases
First, second and third grew from 15 inches square to 18 inches square in 2023.
Three inches sounds trivial. It cuts the distance between first and second by about 4.5 inches, which matters more than it sounds in a sport where stolen bases are decided by fractions.
But the stated reason was safety, not stolen bases. A larger bag gives a runner and a fielder more room to occupy the same spot without colliding, and collisions at first base had been a persistent source of injury. That the change also made steals more attractive was a welcome side effect rather than the purpose.
ABS: the strike zone stops being an opinion
This is the 2026 change, and it is the largest of the five.
Under the Automated Ball-Strike challenge system, a ball or strike call can be challenged and re-decided by an automated measurement of where the pitch actually crossed the zone.
Only the batter, the pitcher or the catcher may challenge. Not the manager, not the dugout — the three people already at the plate, who must decide instantly and without help.
Each team starts with two challenges, and keeps them while it is right. Get two wrong and the team has none for the rest of the game. In extra innings a team is guaranteed at least one, without accumulating unused ones.
Two details are easy to miss. Challenges cannot be used when a position player is pitching — a sensible exclusion, since those innings are not competitive in the usual sense. And where a play draws both an ABS challenge and a video replay review, the ball-strike call is settled first, because everything else on the play depends on it.
The most consequential detail is the zone itself. ABS measures a strike zone scaled to each player’s standing height, and that zone is slightly smaller than the one umpires have called in practice. The system is not merely enforcing the existing zone more accurately; it is enforcing a marginally different zone.
What did not change
The core is untouched. Three strikes, four balls, three outs, nine innings. Ninety feet between bases — the bag got bigger, the basepath did not.
That is the pattern worth noticing. None of these changes altered how a batter gets out or how a run scores. They targeted dead time, defensive extremes, collisions, and disputed judgement — the parts of the sport that had drifted away from what it was meant to look like.
The same instinct produced swimming’s 15-metre rule, which exists because underwater kicking was faster than swimming and the sport decided a swimming race should involve swimming. Rules of this kind are not neutral referees of play. They are the sport defending its own character against optimisation.
Why so much baseball content is out of date
Baseball is not unusual in changing; it is unusual in changing one rule at a time and measuring each. Formula 1’s 2026 reset took the opposite approach and altered the power unit, fuel, aerodynamics, weight and overtaking system in a single season.
Baseball’s rulebook now changes materially most years, and most published explanations of it do not.
Check three things on any baseball rules page: does it mention a clock, does it say 18 seconds, and does it mention challenges? A page that fails any of those is describing a game that is no longer played.
More rules coverage across sports is in rules and techniques.
Frequently asked questions
How long is the pitch clock in MLB?
15 seconds with the bases empty and 18 seconds with runners on. It began in 2023 at 15 and 20 seconds; the runners-on figure was cut to 18 for the 2024 season.
What happens if the pitch clock runs out?
If the pitcher has not begun their motion, an automatic ball is called. If the batter is not ready and alert to the pitcher in time, an automatic strike is called.
Are defensive shifts banned in baseball?
Restricted rather than banned outright. Since 2023 the defence must have two fielders on each side of second base, with both feet on the infield dirt, when the pitch is released.
How big are MLB bases now?
18 inches square, up from 15 inches since 2023. The change shortens the distance between first and second by about 4.5 inches and was introduced primarily to reduce collisions and injuries.
What is the ABS challenge system?
From the 2026 season, a batter, pitcher or catcher can challenge a ball or strike call and have it re-decided by an automated system. Each team starts with two challenges and keeps them while successful.
Does ABS replace the umpire?
No. The home-plate umpire still calls every pitch. ABS only reviews the calls that are challenged, and it applies a strike zone measured to each player's standing height — slightly smaller than the zone umpires have traditionally called.