Baseball, by the Rules
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Called Ball
The pitch comes in, the batter does not swing, and the umpire judges that the pitch did not cross the strike zone. At this point, the umpire will call a ball by… generally doing nothing. Whereas most of the other calls that an umpire makes are also indicated by some sort of hand signal or gesture, the “ball” call is unique in that the indication is a lack of action1.
To call a ball, the umpire must judge that the pitch never intersected the strike zone, not even a little bit. The only exception to this is a pitch that hits the ground and then bounces into the strike zone; as soon as the pitch hits the dirt, it can no longer be called a strike. A hitter can still swing at the pitch and miss, but that would be a swinging strike, not a called strike2.
The way that a catcher receives a pitch can also be an important part of how that pitch is called. If a catcher has to move their mitt too much in the process of making the catch, the umpire might think that the ball missed the strike zone, regardless of its actual location.
Fortunately for a catcher in this situation (or unfortunately for the batter), there is a higher authority that the defense can call upon: the Automated Ball Strike System.