In baseball, the strike zone is a conceptual rectangular area 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 the mid-level between the top of the batter's shoulders and his belt, and the bottom is at the level just beneath the knee cap. The right and left boundaries of the strike zone correspond to the edges of home plate. A pitch at which the batter does not swing and which does not pass through the strike zone is called a ball. Unofficially, the strike zone in Major League Baseball is often enforced as being from the knee of the batter to no higher than his belt, although there are a handful of umpires known to call the 'high' strike.
The strike zone is often illustrated as a two dimensional plane parallel to the front of the plate and perpendicular to the playing surface. If any part of a pitched ball intersects any portion of this plane, the ball is in the strike zone and should be ruled as a strike (unless hit.) Technically, the strike zone has depth as well; the rules define a volume of 3-dimensional space–a right pentagonal prism. If any part of the ball intersects any part of this space, it is considered in the zone, and should be ruled a strike.
A batter who accumulates three strikes in a single batting appearance has struck out and is ruled out (exception, see dropped third strike); a batter who accumulates four balls in a single appearance has drawn a base on balls (or "walk") and is awarded 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.
A strike shall be called and added to the batter's count, when he...
- Swings at a pitched ball and fails to hit it (swing and miss, strike swinging). According to MLB Rule 2.00 Definition of Terms, STRIKE (a), a pitch is called a strike if it "[i]s struck at by the batter and is missed".  This includes when he:
- Attempts to bunt a pitched ball and fails to hit it.
- Touches a pitched ball with his body while striking at it with the bat.
- Fails to swing at a pitched ball which is called a strike—determined to be in the strike zone—by the umpire. (called strike, strike looking)
- Hits a pitched ball into foul territory when there are fewer than two strikes in the at-bat. (foul ball)
- Bunts a pitched ball into foul territory. This counts as a foul strike regardless of the number of strikes already charged to the batter.
- Touches a pitched ball while it is in the strike zone. (Intentional touching of a pitched ball is not allowed; see hit by pitch.)
- Hits a foul tip; that is, ticks the ball which goes directly to the catcher's glove and is caught.
- Refuses to enter the batter's box when ordered to do so by the umpire, and any directed pitch is thrown by the pitcher.
A normal foul strike cannot count against the batter as his third strike; the third strike must be a swing and miss, called strike, touched ball, foul bunt or foul tip.
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. Umpires often call pitches according to a contemporary understanding of the strike zone rather than the official rulebook definition. The conventional definition that prevails in Major League Baseball shifts the whole strike zone laterally a few inches away from the hitterwhile truncating the zone vertically near the batter's belt.
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 evalulation has brought today's strike zone closer to the rulebook definition is a matter of debate.
Many factors have contributed to the divergence of the official and conventional strike zones. 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. As pitchers lost the higher strike zone, they began throwing lower and to the outside, which caused hitters to move closer inside.
At the same time, there was a shift in attitude among both players and league officials regarding pitches thrown inside. While pitchers of the 1960's 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.
Despite the fact that the conventional 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.