The strike zone is an imaginary 3D rectangle above home plate that is the width of home plate and the height between the batter's knee and chest. If the pitch is inside the strike zone, it is a strike; if it is outside the strike zone, it is a ball. The home plate umpire determines whether or not a pitch is in the strike zone.
Determined from a batter's stance, the strike zone is a concept, and each umpire has a different idea of what it conceptually looks like in his or her head.
How The Strike Zone Works In Games
A pitcher begins each at-bat with pitch, where he throws the ball to the catcher at home plate. Every pitch is either a strike or a ball. The umpire to judge the pitch uses an imaginary box called the strike zone. The strike zone is the width of home plate and it extends from just below the batter's knees to the middle of the top of his pants and shoulders. Umpires will track the pitch as it's thrown from the pitcher's mound to the catcher observing how it travels through the strike zone. A pitch thrown through the strike zone is a strike, and a ball thrown outside. 4 Balls and the batter is walked or earns a free base-on-balls. 3 strikes and the batter has struck out, giving the defense 1 of 3 outs to end the half-inning.
The umpire also determines strikes based on how the batter reacts to the pitch and where it ultimately ends up. Strikes are always called when a batter swings and misses. So, if a batter swings at a pitch he essentially the umpire disregards the strike zone. A ball can never be called when a batter takes a swing. So even if the ball was thrown outside the strike zone it's a strike if he misses. All the credit to the batter if he can successfully hit the ball on a pitch outside the strike zone, a decision he must make.
When a batter is at-bat his goal is to make it safely on base and eventually score. He is given a series of pitches where the pitcher throws the ball to the catcher at home plate. Every pitch is either a strike or a ball.