Before the Automated Ball-Strike System is ready for the major leagues, there has to be agreement on what a strike is.

"You go in a rabbit hole where it might not be fair for some hitters," Minnesota Twins star Carlos Correa said. "A pitcher has big curveballs that cross the zone and end up a ball, but the zone has it as a strike. I prefer the human element of things, but who knows, maybe they can perfect it at some point."

MLB started experimenting with robots calling balls and strikes in the independent Atlantic League in 2019 and used the computer at Low-A in 2021. A challenge system was tried last season at some minor league ballparks, in which a pitcher, batter or catcher had the right to appeal a human umpire's decision to the computer. This year, ABS is being used at all Triple-A parks, the robot alone for the first three games of each series and a human with a challenge system in the final three.

The Official Baseball Rules define the strike zone as "that area over home plate the upper limit of which is a horizontal line at the midpoint between the top of the shoulders and the top of the uniform pants, and the lower level is a line at the hollow beneath the kneecap."

In practice, big league umpires usually don't call strikes on pitches that clip the bottom of the three-dimensional zone's front or the top of the back, making the actual strike zone more of an oval than a cube.

"If you looked good at the average zone that's called in the major leagues and has been called forever, it's not a rectangle the way that the system calls it," MLB executive vice president of operations Morgan Sword said.