In baseball, the strike zone is a conceptual three dimensional right angle 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 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. Unofficially, the de facto enforced strike zone may be different at any different level.
Without further official guidance to supplement the Strike Zone definition in the MLB Official Rules, the following two decidedly un-official declarations may capture the two opposing extremes of allowable interpretation for a "Pitch-Not-Struck" to qualify as a strike under the rules:
(1) No such pitch shall be a strike unless an entire diameter of the ball—including stitched seams—has passed inside the area formed by all strike zone boundaries as defined in the official rules. [Favors Batter]
(2) Each such pitch shall be a strike if any portion—no matter how small, including stitched seams—or all of the ball has intersected or passed inside any strike zone boundary as defined in the official rules. [Favors Pitcher]
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 like 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, Major League Baseball began privately evaluating umpires based on the QuesTec pitch-tracking system. Whether such evaluation has brought today's strike zone closer to the rulebook definition is a matter of debate, but 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.
Despite the fact that the enforced strike zone is a departure from one of the fundamental rules of baseball, the difference does not garner a great deal of attention. In general, players and managers consider consistency rather than accuracy to be the most important characteristic of a well-judged strike zone. Often in amateur or youth games, the enforced strike zone may differ greatly from umpire to umpire; participants typically expect the umpire to be consistent within the game only.