For more than a century, baseball’s home plate umpires have called a ball or strike based on interpretation of a vague, loosely defined strike zone.
These subjective calls decided at-bats, games, seasons and pennants — and, naturally, stirred endless debate.
Now, for the first time, this season Major League Baseball is instituting a review system in which players can challenge a ball-strike call. Meaning: For the first time the strike zone will be defined and there will be a definitive answer to the debate.
The Automated Ball-Strike (ABS) challenge system, which has been tested in the minor leagues and MLB spring training, will make its debut Wednesday night in the MLB season opener, when the San Francisco Giants play host to the New York Yankees — coincidentally in America’s high tech capital.
After every pitch, when the umpire calls ball or strike, there will be a two-second window in which the batter, pitcher or catcher can initiate a challenge.
Once a review is requested, the stadium scoreboard will show the system’s ruling on whether the pitch passed through the strike zone. Everyone will have to wait on bated breath, similar to tennis and challenges of in/out calls.
Each team will get two challenges a game, and will keep them if they’re right.
But in order for baseball to create its system, MLB had to definitively define its strike zone, which has been a moving, subjective target since the dawn of the game.
As far as rule changes go, this is a massive shift for baseball, expected to change gameplay and strategy, not unlike when the American League adopted the designated hitter in the 1970s.
“I think [the ABS system] is more important than the DH, and even more than the foul-strike rule at the turn of the last century,” MLB historian John Thorn told NBC News.
What is the new strike zone?
Before Wednesday night, for all of baseball’s history, the strike zone had been very much subjective, up to the judgment of the umpire. A strike had most recently been defined as the zone over home plate, bordered at the top as halfway between a “batter’s shoulders and the top of the uniform pants” and then at the bottom at “a point just below the kneecap.”
“Umpires traditionally have called something that is kind of flexible, right? It depends on count, game situation, score, all those things,” MLB vice president of on-field strategy Joe Martinez said. “The strike zone tends to expand and contract based on those types of things.”
Now, when a ball-strike call is challenged, the strike zone will be rigidly defined based on a batter’s height. A strike is now defined as a pitch that sails over home plate and that’s between 27% and 53.5% high of a hitter’s height.
Under the new system, if a challenged pitch clips any part of this imaginary “pane of glass” that’s virtually based in the middle of home plate, that’ll be called a strike, Martinez said.
What are the stakes?
Why does baseball need replay review?
Look no further than the recent World Baseball Classic. In a semifinal between the United States and the Dominican Republic, the game ended on a called third strike, on a pitch that appeared to be well out of the zone.
That pitch “was 3 inches below the zone, [a replay system] could have changed everything,” Miami Marlins catcher Liam Hicks said. “So hopefully [ABS] saves some big moments from going the wrong way. So I think I’m a fan of it.”
But even a missed call that’s not a ball four or strike three can have a profound impact. For example, when the count is 2-1 and the batter doesn’t swing at a borderline pitch, his fate will be dramatically shifted by the ensuing call.
If a ball is called and the batter goes ahead of the count 3-1, that hitter in 2025 went on to have a .255 batting average, a .592 on-base percentage, a .453 slugging percentage and a 1.045 OPS. Or, in other words, that ordinary hitter, with a 3-1 edge, became the equivalent of all-world slugger Shohei Ohtani, who recorded a 1.014 OPS last season.
But if that umpire calls a strike and the count levels at 2-2, everything flips to the pitcher’s edge. That hitter, at 2-2 last year, went on to have a microscopic .178 batting average, .286 on-base percentage, an anemic .291 slugging percentage and a .577 OPS.
Of all MLB batters last year with enough qualified plate appearances, the lowest OPS was recorded by Milwaukee Brewers shortstop Joey Ortiz (.230/.276/.317) at .593.
What is the strategy?
Interestingly, the players will have to decide whether to challenge on their own. They can’t take guidance from coaches, teammates or rogue fans screaming from the stands.
A player’s “decision to challenge has to be unaided” and “if anyone’s yelling from the dugout, if another player is on the field, tapping his head, umpires have the ability to deny a challenge,” the MLB executive Martinez said.
Each team can keep challenging until it’s wrong twice, so these valuable appeals shouldn’t be employed willy-nilly.
Miami Marlins manager Clayton McCullough said his team will dissuade its pitchers from calling for a challenge, even though they are allowed, leaving the decision to the catcher. The pitcher stands about 60 1/2 feet away from home plate after all, much further than the catcher.
“Our pitchers are not going to be able to challenge,” McCullough said. “We’ll put a lot of stock in all of our catchers being very good at doing this. It’s led us to creating training environments for them in camp, our catcher specifically, to spend a lot of bandwidth on training with an automated strike zone.”
In triple-A last year, challenges were 50% successful, and they were 51% successful the year before that.
This spring, most major league players and managers said they planned to save appeals for late innings and other key points of the game, known as “high leverage” situations.
In triple-A games in 2025, the highest percent of challenges came in the ninth inning (3.5% of all called pitches), and the least came in the first inning (just 2.1%).
“[What] we’ve tried to stress to them during this camp is, yes, we’d love to be the most successful at overturning calls,” McCullough said. “Also, probably more importantly, we want to be challenging at the most appropriate times.”
But counterintuitively, first-year San Francisco Giants manager Tony Vitello said he won’t mind his players being more aggressive in using challenges even early in the contest.
“I think the biggest thing is you don’t want to leave them on the table,” said Vitello, a highly successful coach at the University of Tennessee who is believed to be the first MLB manager with no previous big league experience.
“If you see something that you feel like, ‘Nah, I know for sure that’s off,’ you might as well give it a go.”
Learning curve for fans
At a spring training game last week between the St. Louis Cardinals and Miami Marlins, many fans reached by NBC News had no idea that MLB had installed this challenge system for 2026 — even though a pregame tutorial was played on the scoreboard at Roger Dean Chevrolet Stadium in Jupiter, Florida.
“I’m a traditionalist and I don’t like it,” said Dallas resident Heather Garrison, who was at the game to see her son, Marlins bullpen catcher Tanner Garrison. “I like the old way where you trust that they’re going to make a call and you go with it and in the end, it all kind of comes out even.”
Another fan in Jupiter that day, Jordan Waxman, said he would prefer any challenge ruling be made by a human, rather than a computer, such as in the case of safe/out calls reviewed at the MLB replay center in New York.
“I don’t like [the new replay system], I don’t like anything to do with AI,” Waxman said. “The game’s 140-plus years old and they never needed it [an automated system] until now. It just takes away from the history of the game. I prefer them calling New York for a challenge than it being done computerized. You start getting computers involved, then what’s next?”