From BR Bullpen
The strike zone is defined in the official Major League Baseball rules 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 knee cap. The Strike Zone shall be determined from the batter's stance as the batter is prepared to swing at a pitched ball."
 Related Sites
- The Definition of the strike zone in the oficial MLB rules: definition
- Article on the difference between American League and National League strike zones in the 1940s and 1950s in The Hardball Times