Baseball, by the Rules
Cal Raleigh successfully challenges a pitch by Tarik Skubal during the 1st inning of the 2025 All-Star Game.
The ABS Challenge System, Explained
The “ABS” in ABS Challenge System stands for “Automated Ball Strike”, and refers to a system by which a number of high-speed cameras track a pitch in flight, then determine whether or not that pitch was a strike, potentially overriding the call of the home plate umpire who initially called that pitch. The “Challenge System“ part refers to how ABS has been implemented by Major League Baseball. Put it all together1, and you get the ABS Challenge System! This is sometimes referred to as “robot umpiring,” but there aren't any robots involved... yet.
The basics of the ABS Challenge System are that all balls and strikes are initially called by the home plate umpire, which is how baseball has worked, more or less, for around 170 years. But! Each team starts the game with two challenges that they can deploy. After a called strike, if the batter thinks the call was wrong, they can challenge that call. Similarly, if after a called ball, the pitcher or catcher thinks the call was wrong, they can challenge that call. No one else has the ability to challenge a call: only the pitcher, the catcher, and the batter. The player in question initiates a challenge by tapping on their head, and they have to do so “immediately” after the call is made. According to MLB, “immediately” generally means “within around two seconds of the pitch,” though in cases where the pitch is followed by some sort of play on the bases, the challenge can still be issued after said play is over2. Oh, and if whatever play also involves a call that may get challenged, the unmpires are supposed to let the ABS challenge go through before starting the replay. Indeed, if a play is reviewed, no ABS challenges can be made after that review.
Once a challenge is made (and accepted)3, the umpire signals the video crew, who displays the ABS replay of the pitch, which shows where the ball wound up. The ABS' judgement is final, so if the pitch is shown to have hit the strike zone, even a little bit, the result is a strike. Otherwise, it's a ball. This may or may not mean that the original call gets overturned; either way, the game proceeds from that point. If the player who made the challenge was wrong, their team loses their challenge. If they were right, they keep said challenge. In other words, if an umpire calls a strike, the batter challenges, and the result is a strike, the batter's team is now down a challenge. If a team runs out of challenges before the game ends, no more challenges— unless the games goes to extra innings. In each extra inning, if a team has run out of challenges, they get a new challenge to work with. These challenges do not roll over, so a team won't be able to go from, say, zero challenges in the 9th to three challenges by the time the 12th rolls around.
Hopefully that all seems easy enough to grasp, but there's one more wrinkle: the strike zone that shows up in the rulebook is slightly different from the strike zone that ABS calls. The main difference is in the dimensions. The rulebook strike zone is three-dimensional, with the X-axis being the width of the plate, the Y-axis being the height of the batter, and the Z-axis being the depth of the plate. ABS, however, tracks a two-dimensional zone, dropping that Z-axis and focusing on a single sliver cut midway through the plate. The Y-axis it measures is subtly different, as well. According to ABS, the top end of the zone is at 53.5% of the player’s height, while the bottom is at 27% of the player’s height. It all adds up to a single-planed strike zone. If ABS judges that a pitch doesn't intersect that plane, it's not a strike.
The ABS Challenge System was tested heavily in the minor leagues4, prior to being implemented for the 2026 MLB season. As we are writing this before said 2026 season, it's impossible to say what sort of effects this change will have, though it seems likely that both walk and strikeout rates will go up, at least initially5. MLB will also likely iterate on the system in further seasons; for example, one early note seems to be that the “tap your helmet” signal can either be misinterpreted, leading to accidental challenges, or missed entirely. On the other hand, sometimes batters will fire off a challenge and then immediately regret it, which is its own fun.
Oh, and one last thing: the ABS Challenge System will only see use in games played in major league stadiums, which feels like it might be self-explanatory, but a typical MLB season tends to include one or two special events played in special venues6, and those games will be played without the ABS tracking system in place.
Acknolwedgements |