Definition
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.
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