In baseball, the strike zone is the volume of space through which a pitch must pass in order to be called 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 passes through the zone 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 if the batter doesn't swing. Balls are desirable for the batter and the batting team, as four balls allow the batter to take a "walk" to first base as a base on balls.
Multiple sets of rules govern baseball and softball, which define the strike zone slightly differently. The rulebook in use depends on the level and league.
The Major League Official Rules defines the top of the strike zone 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 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.
The strike zone is 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".
Major League Baseball has occasionally increased or reduced the size of the strike zone in an attempt to control the balance of power between pitchers and hitters. After the record home run year by Roger Maris in 1961, the major leagues increased the size of the strike zone from the top of the batter's shoulders to the bottom of his knees. In 1968, pitchers such as Denny McLain and Bob Gibson among others dominated hitters, producing 339 shutouts.Carl Yastrzemski would be the only American League hitter to finish the season with a batting average higher than .300. In the National League, Gibson posted a 1.12 earned run average, the lowest in 54 years, while Los Angeles Dodgers pitcher Don Drysdale threw a record 58 and two-thirds consecutive scoreless innings during the 1968 season. As a result of the dropping offensive statistics, Major League Baseball took steps to reduce the advantage held by pitchers by lowering the height of the pitcher's mound from 15 inches to 10 inches, and by reducing the size of the strike zone for the 1969 season.
Although the de facto enforced strike zone can vary, the Official Rules (Definitions of Terms, STRIKE (b)) define a pitch as a strike "if any part of the ball passes through any part of the strike zone."
A batter who accumulates three strikes in a single batting appearance has struck out and is ruled out (with the exception of an uncaught third strike); a batter who accumulates four balls in a single appearance has drawn a base on balls (or walk) and is awarded advancement to first base. In very early iterations of the rules during the 19th century, it took up to 9 balls for a batter to earn a walk; however, to make up for this, the batter could request the ball to be pitched high, low, or medium.
While baseball rules provide a precise definition for the strike zone, in practice, it is up to the judgment of the umpire to decide whether the pitch passed through the zone.
The Official Baseball Rules (Rule 8.02(a), including Comment) state that objections to judgment calls on the field, including balls and strikes, shall not be tolerated, and that any manager, coach, or player who leaves his dugout or field position to contest a judgment call will first be warned, and then ejected.
Many umpires, players and analysts, including the authors of a University of Nebraska study on the subject, believe that due to the QuesTec pitch-tracking system, the enforced strike zone in 2002-2006 was larger compared to the zone in 1996-2000 and thus closer to the rulebook definition. Some commentators, such as Tim Roberts of covers.com, believe that the zone has changed so much that some pitchers, such as Tom Glavine, have had to radically adjust their approach to pitching for strikes. In 2003, a frustrated Curt Schilling took a baseball bat to a QuesTec camera and destroyed it after a loss, saying the umpires shouldn't be changing the strike zone to match the machines.
In 2009, a new system called Zone Evaluation was implemented in all 30 Major League ballparks, replacing the QuesTec system; the new system records the ball's position in flight more than 20 times before it reaches home plate. Much of the early resistance from Major League umpires to QuesTec had diminished and the implementation of the new Zone Evaluation system in all the parks went largely unnoticed. Like the old system, the new system will be used to grade umpires on accuracy and used to determine which umpires receive postseason assignments.