Baseball is unique in that one of the central and most important features of the field of play is completely invisible. There are no lines that mark it, no buzzers or bells that go off when it’s touched. There is a five-sided plate in the ground beneath it, but that’s more for the purpose of having a base to touch when a runner scores than to define the strike zone itself. Although it does, of course, contribute to the definition of the strike zone.

How wide it the strike zone?

You may know that home plate is 17 inches wide. So if you’re at a bar with friends and someone bets you a beer if you can answer this question, you may be tempted to reply (maybe smugly): “seventeen inches.” Bzzzzzz. Wrong. You owe your buddy a beer.

Let’s start with the obvious:  the rule book: Definitions of Terms (strike zone) (OBR 2016):

The STRIKE ZONE is 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. The Strike Zone shall be determined from the batter’s stance as the batter is prepared to swing at a pitched ball.

So the strike zone is a three-dimensional area over the plate (a rectangular prism for you geometry geeks), and it extends from the hollow at the bottom of the knee to a point “at the midpoint between the top of the shoulders and the top of the uniform pants.” Discussing the top and bottom of the zone is a discussion all to itself (because it fluctuates depending on many factors), so let’s save that for another post. For now, let’s concentrate on the “over the plate” part of the definition. But wait. First, we have to ask what it means to be “over the plate.”

To answer this, we turn again to rule book definitions, but this time we’re looking for the definition of a strike, which we find in Definitions of Terms (strike), where we learn, among other things, that a pitch is a strike “… if any part of the ball [in flight] passes through any part of the strike zone.” In other words, if any part of the ball touches any part of the strike zone, it is, by rule, a strike. (Note that I added that the ball must be “in flight”; that is, a pitch cannot be a called strike if it first touches the ground then bounds through the strike zone.)

So here’s what we know:

  • Home plate is 17 inches wide (Rule 2.02)
  • A regulation baseball is just a shade under three inches in diameter (Rule 3.01). (In deference to the geeks among us, the diameter is actually 2.9443 inches; but what’s sixty-six hundredths of an inch among friends. So let’s just call it three.)
  • If any part of the ball in flight touches any portion of the strike zone, it’s a strike.
  • Therefore, we see that the strike zone is 23 inches wide.

Note that the black on the edge of the plate is not part of the plate.

In truth, the “over the plate” part of judging balls and strikes is the (relatively) easy part. The top and bottom of the zone, on the other hand, is a hotly contested, furiously debated, and nearly impossible to pin down aspect of the game. While defined by rule, it is adjudicated by eye, and we all know where that leads: arguments and ejections. I saw a spring training game earlier this week between the Yankees and Braves in which the Braves manager Fredi Gonzalez was ejected by home plate umpire Dan Iassogna for arguing a ball call on the game’s very first pitch. Holy guacamole, Batman! (And by the way, Gonzalez was right. It was a strike.)

We are going to talk a lot more about the strike zone over the coming months and years. We’re going to discuss the perilous subject of the top and bottom of the zone, as well as the mysterious art of calling balls and strikes. We’re going to talk about Pitch f/x data, on variations among umpires, differences for left- and right-handed hitters, differences related to the current count (how does an 0-2 strike differ from a 3-0 strike, for example), and a great deal more.

So stay tuned. The strike zone is a marvelous (if invisible) part of the game of baseball, and there is no end to trouble we can create by discussing its mysterious contours.