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(Photo by Caitlin O’Hara/MLB Photos via Getty Images)
This year’s spring training has given fans a reason to think about the strike zone in more detail than they’ve ever done before. And the experimental use of the automated ball-strike (ABS) challenge system has led to a lot of fans wondering why the strike zone being used by robot umpires doesn’t match the one they’ve seen in the rule book.
Now, this is a part of baseball that has usually bubbled beneath the surface. You may know what should be a strike and what’s a ball. But if asked to define it, it gets complicated quickly.
If you have ever used the term “hollow beneath the kneecap” in a sentence, you’ve definitely read the MLB rule book. If you’ve ever given thought to where the midpoint between the top of the shoulders and the top of the uniform pants truly resides, you’ve likely umpired games at some point.
But as MLB has edged closer and closer to bringing the ABS challenge system to big league baseball, the success of the system has depended on putting that rule book definition back on the bookshelf.
Whenever MLB has tried to match the rule book definition of the strike zone in its many MiLB, Arizona Fall League and Atlantic League experiments, it’s frustrated batters, pitchers, umpires and fans. By being strict to the rules, the ABS zone seemed to be out of whack.
Only by being willing to be unconventional has MLB gotten to the point where the robo-ump can call what we perceive as the conventional strike zone.
The strike zone is exceptionally complicated. It has moved around from era to era. But for everyone who watches a lot of baseball, you get acclimated to the “normal” zone pretty quickly.
What is “normal” will change.
In the 1945 MLB rule book, the strike zone was defined as being “not lower than the batsman’s knees, nor higher than his shoulder.” Imagine a modern day batter getting called on a high fastball at collarbone height.
By 1956, the definition in the official rule book stated that it was the space “between the batter’s arm pits and the top of his knees when he assumes his natural stance.” Now we have the “hollow beneath the kneecap” and the midpoint. So even when it comes to the rulebook definition, the zone has changed over the years.
In practical terms, it is always changing. There was a time where the American League and National League zones were different because of the equipment they used.
Until the late 1970s and early 80s, AL umpires used external balloon chest protectors, while NL umpires used the chest protectors of the style still used today. That meant that the National League zone called more low strikes than the American League zone, because using the bigger balloon protector didn’t allow umpires to get as close or low in their stance.
All those ideas you may have to make the ABS strike zone more accurate? They’ve been tried.
When the ABS system first came to the Atlantic League in 2019, it used a three-dimensional zone. But that didn’t last long, because everyone quickly realized that thinking of the zone as a 17-inch-long box allowed too many breaking balls to barely nick the top, bottom or sides of the zone. A backdoor breaking ball clearly off the plate could curve in just enough. That led to pitches being called strikes that left batters fuming and fans flummoxed.
Now, the zone is a two-dimensional box set at the halfway point of the plate (measured from front to back). There are other options that have been considered. The KBO, for example, has experimented with using a pair of boxes, with the pitch having to touch both boxes to be considered a strike. The rectangular zone could be set at a slant instead of being perpendicular to the ground. That would account for gravity where the top of the zone is further forward than the bottom of the zone.
But any of those ideas adds complexity to something that ideally needs to be easy for fans to understand at a glance. The prevalence of “k-zone” graphics on MLB broadcasts has already conditioned fans to think of the strike zone as a two-dimensional rectangle. Multiple MLB-led experiments over the past six years have found that this simple approach seems to better replicate the expected strike zone for players and umpires.
When MLB and the Atlantic League first experimented with an ABS zone in 2019, it used a definition of the strike zone that was pretty close to the rule book. Hitters went crazy after getting called on high pitches they had never seen counted as strikes. Within a week, the zone was moved down a few inches, because, again, the textbook definition isn’t what any umpire calls.
Basing the height of the strike zone off a live representation of the hitter’s stance at the start of the pitch? That was tried in Triple-A. Player surveys found it was preferred to have the zone based off a player’s height. Testing found that umpires don’t adjust their zones significantly for batting stances.
Early on, MLB tried tweaking the zone to try to create more action and reduce strikeouts. Even through multiple iterations—making the zone wider and shorter, reducing some of the top of the zone for fewer high-fastball strikeouts—nothing really worked. Most led to more walks without creating more balls in play. It also led to too many weird calls.
Eventually, the league realized that it’s best bet was to try to make an automated zone that faded into the background. Ideally, hitters, pitchers and fans should see calls by the ABS as “normal.”
And in that, it seems like the ABS challenge system is succeeding. Several weeks into the spring training experiment, other than some complaints from Max Scherzer, the system has been largely unnoticeable.
This has been MLB’s way with major rules changes in recent years.
Over the past five years, multiple significant rules changes have been adopted, including pitch clocks, bigger bases, shift limits and limits on pickoff throws. In all cases, the bugs, problems and mistaken experiments have been cleared up well before the changes arrive in the majors.
We’re seeing the next tests start bubbling up as well, as automated systems to rule on check swings were tested last October in the Arizona Fall League.
But the potential arrival of the ABS challenge system in the major leagues for 2026 is likely the next big step. At this point, the bugs have been squashed. There still has to be a decision on whether to bring it to the major leagues, but the tech is ready.
And when it arrives, it will lead to a slightly-different understanding of the strike zone. But that’s a never-ending part of baseball.
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