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.
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.
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.
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