In baseball, the strike zone is a conceptual right pentagonal prism over home plate which defines the boundaries through which a pitch must pass in order to count as a strike when the batter does not swing.
The top of the strike zone is defined in the 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. 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.
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. 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 pitchers mound from 15 inches to 10 inches, and by reducing the size of the strike zone for the 1969 season.
The de facto enforced strike zone can vary. An extreme interpretation that favors batters requires the entire diameter of the ball—including stitched seams— to pass inside the area formed by the strike zone boundaries as defined in the official rules. The opposite extreme—favoring pitchers—requires a pitch to be called a strike if even the smallest portion of the ball, seams included, has intersected or passed inside any strike zone boundary as defined in the official rules.
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. Historically, umpires often call pitches according to a contemporary understanding of the strike zone rather than the official rulebook definition.
Many factors have contributed to the divergence of the official and conventional strike zones in Major League Baseball. Changes began in the 1970s, when umpires upgraded their chest protection in favor of more compact vests allowing them more movement. Crouching lower meant lowering their line of vision, and caused the boundaries of the strike zone to sink lower. Thus, the strike zone was often enforced such that pitches above the waist were balls, and pitches a few inches outside of home plate were called strikes. As pitchers lost the higher strike zone, they began throwing lower and to the outside, which caused hitters to move much closer to the plate.
At the same time, there was a shift in attitude among both players and league officials regarding pitches thrown inside. While pitchers of the 1960s such as Bob Gibson regarded it a pitcher's right to throw high and inside, later batters were more likely to take offense at such treatment. Major League Baseball also tightened its rules prohibiting pitchers from intentionally hitting batters, removing the warning pitchers formerly received before being ejected from a game. Soon, hitters moved closer to the plate and looked for the ball outside.
In 2001, Major League Baseball directed its umpires to call pitches according to the official definition rather than the conventional one. Umpires were to call "high" strikes and "inside" strikes, while pitches just off the outside part of the plate were to be called balls. The umpires demonstrated limited compliance for a time, but before long the de facto strike zone had returned to the conventional definition. Shortly thereafter,[specify] Major League Baseball began privately evaluating umpires based on the QuesTec pitch-tracking system. Most umpires, players and analysts, including the authors of a University of Nebraska study on the subject, believe that due to QuesTec, 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 post season assignments.
Further reading 
- Gammons, Peter (April 6, 1987). "What Ever Happened to the Strike Zone?". Sports Illustrated 66 (14): 36–40, 45–46.
See also 
- "1968: Year of the Pitcher". thisgreatgame.com. Retrieved 25 December 2011.
- "Expanded strike zone unveiled". The Press-Courier. Associated Press. 8 March 1963. p. 9. Retrieved 25 December 2011.
- "McLain Says Lower Mound Will Take Toll of Pitchers". The Telegraph-Herald. Associated Press. 14 January 1969. p. 13. Retrieved 25 December 2011.
- Newswise Social and Behavioral Sciences News | Larger Strike Zone, Drug Testing Reduced Hitting in Baseball Since 2000
- Umpires and totals: Men behind the mask occasionally steal the show
- D'backs' Schilling fined for destroying QuesTec camera
- Monitor May Reopen Wounds, an April 2009 article from The New York Times
- Preview 2009: The umpires' arbiter from an April 2009 Star Tribune article
- Zimniuch, Fran (2010). Fireman: The Evolution of the Closer in Baseball. Chicago: Triumph Books. p. 75. ISBN 978-1-60078-312-8.
- 2001 Changes in Strike Zone - St. Petersburg Times article.
- Strike Zone MLB website.
- John Walsh, "Strike Zone: Fact vs. Fiction," The Hardball Times, July 11, 2007.
- The Strike Zone: A Chronological Examination of the Official Rules .