When a pitcher pitches the baseball to home plate to the catcher, the pitch will be called as either a strike or a ball by the Umpire. It's all determined by an imaginary zone at home plate called the strike zone.
The Strike Zone
The strike zone is an invisible rectangle right above home plate that is the width of home plate and extends from the batter's knees to his chest. It is used to by the Umpire to determine if a pitch is a ball or a strike. It is up to the Umpire to create a mental picture of the strike zone in his mind when calling a pitch.
The strike zone is considered a hittable area; that is, pitches that are within the strike zone are considered pitches that the batter can realistically hit. This is important to note because it defines the strike zone's purpose. If the batter does not hit a pitch that can be realistically hit/is in the strike zone, he is punished with a strike (which can later lead to an out). If the pitcher throws a pitch that cannot be reasonably hit/is outside the strike zone, the batter is awarded with a ball (which can later lead to a walk). We will learn more about balls and walks later in this chapter.
The Umpire's Role
The home plate umpire ultimately makes that decision though. He stands behind the catcher and tracks the path of the pitch as it travels from the pitcher's mound to the catcher's glove. If he sees that the pitch was inside the strike zone, he will call it a strike. If he sees that the pitch was outside the strike zone, he will call it a ball.
Human Error When Calling Strikes And Balls
However, umpires are human and humans are not perfect. Fans, players, and coaches become upset if they feel that an umpire has wrongly called a pitch or has an atypical strike zone. While ideally an umpire's call is correct every time, a pitch that is inside or outside the strike zone will not always be called as such. Human error of the umpires is an important aspect of baseball, and it affects many areas of the game -- including the strike zone.
When the Strike Zone Is/Is Not Used
In some situations, the strike zone is disregarded and a strike is called because of something else. The strike zone is only used when the batter does not swing at the pitch. Not swinging at the pitch means that the batter did not move his bat as the pitch went by, or that the batter performed a check swing. Check swinging is when the batter begins his swing, but the bat does not cross home plate. Check swinging counts as not swinging at all, and the strike zone is still used to determine whether the pitch is a ball or a strike. Umpires at first and third bases can help the home plate umpire decide if the pitch was a check swing, so the home plate umpire can use the strike zone accordingly.
If the batter does swing his bat, the strike zone is not used and the outcome of the pitch/play is determined by the outcome of the swing. In the next chapter, we will learn more about how strikes are called without using the strike zone.