MLB Strike Zone
The official strike zone is the area over home plate from the midpoint between a batter's shoulders and the top of the uniform pants -- when the batter is in his stance and prepared to swing at a pitched ball -- and a point just below the kneecap. In order to get a strike call, part of the ball must cross over part of home plate while in the aforementioned area.
1957 - "A strike is a legal pitch when so called by the umpire which (a) is struck at by the batter and is missed; (b) enters the Strike Zone in flight and is not struck at; (c) is fouled by the batter when he has less than two strikes at it; (d) is bunted foul; (e) touches the batter as he strikes at it; (f) touches the batter in flight in the Strike Zone; or (g) becomes a foul tip. Note: (f) was added to the former rule and definition."
1907 - "A fairly delivered ball is a ball pitched or thrown to the bat by the pitcher while standing in his position and facing the batsman that passes over any portion of the home base, before touching the ground, not lower than the batsman's knee, nor higher than his shoulder. For every such fairly delivered ball, the umpire shall call one strike."
1894 - "A strike is called when the batter makes a foul hit, other than a foul tip, while attempting a bunt hit that falls or rolls upon foul ground between home base and first or third bases."
In baseball, the strike zone is the volume of space through which a pitch must pass in order to be called a strike even 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.
The strike zone is a volume of space, a vertical right pentagonal prism. Its sides are vertical planes extending up from the edges of home plate. In Major League Baseball, the top of the strike zone is the midpoint between the top of the batter's shoulders and the top of the uniform pants, and 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. Various rulebooks for baseball and softball define the strike zone slightly differently.
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", with the ball required to have not bounced. Thus, 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 passing outside the front of the strike zone but curving so as to enter this volume farther back (without being hit) is called a "back-door strike".
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.
Originally, the word "strike" was used literally: the batter striking at the ball in an effort to hit it. For example, the 11th of the Knickerbocker Rules (1845) read "Three balls being struck at and missed and the last one caught, is a hand-out." There was no adverse consequence if the batter chose not to swing, i.e. th