Strike zone

In baseball, the strike zone is the volume of space through which a pitch must pass in order to count as a strike (if the batter does not swing). The strike zone is defined as the volume of space above home plate and between the batter's knees and the midpoint of their torso. Whether a pitch passed through the zone or not is decided by an umpire, who is generally positioned behind the catcher.

Strikes are desirable for the pitcher and the fielding team, as three strikes result in a strikeout of that batter. A pitch that misses the strike zone is called a ball. Balls are desirable for the batter and the batting team, as four balls allow the batter to take a base on balls.

There is more than one set of rules that govern baseball and softball. It depends on the level and league as to which set of rules are being used. The governing bodies for the different sets of rules may have slightly different definitions.

The top of the strike zone is defined in the Major League Official Rules as a horizontal line at the midpoint between the top of the batter's shoulders and the top of the uniform pants. The bottom of the strike zone is a line at the hollow beneath the kneecap, both determined from the batter's stance as the batter is prepared to swing at the pitched ball. The right and left boundaries of the strike zone correspond to the edges of home plate. A pitch that touches the outer boundary of the zone is as much a strike as a pitch that is thrown right down the center. A pitch at which the batter does not swing and which does not pass through the strike zone is called a ball (short for "no ball"). The active tally of strikes and balls during a player's turn batting is called the count.

In practice, the strike zone is treated as a volume of space delimited by vertical planes extending up from the pentagonal boundaries of the home plate and limited at the top and bottom by upper and lower horizontal planes passing through the horizontal lines of the definition above. This volume thus takes the form of a vertical right pentagonal prism located above home plate. A pitch passing outside the front of the defined volume of the strike zone but curving so as to enter this volume farther back (without being hit) is described as a "back-door strike".

...
Wikipedia