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. (For diagram of STRIKE ZONE see page 23.)
The diagram presented in the OBR looks like this:
However, the strike zone is actually three-dimensional, and is better represented by this picture:
While one definition of a Strike is "any part of the ball passes through any part of the strike zone," [Rule 2.00 STRIKE] the truth is that a called strike is largely based on the plate umpire's interpretation. Professional umpires are taught that the ball must substantially pass through the strike zone. That means that a slow "12-6" curve ball that nicks the top of the strike zone at the back part of the plate should not be called a strike, even through it meets the rule book definition of a strike.