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.