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There's a good reason the catcher is often referred to as the general on the field. He must be hyper-aware of his surroundings. His teammates' strengths and weaknesses, as well as the opponents', are all knowledge that a catcher must have ingrained. The more a catcher knows about what is going on with everyone around him, the more likely he is to come up with good and quick decisions.
The catcher must have an extraordinary working knowledge of everyone on the opposing squad, whether they are found in the starting lineup or not. It would be dangerous to call for pitches without this sort of intel at his fingertips. What speeds and pitch types a batter is or is not fond of must be within a catcher's immediate arsenal. Truly good catchers have an even more extensive knowledge, knowing a batter's habits based on different pitch counts and game situations. When you consider how many opposing teams a catcher will see in a single season, it becomes an almost baffling amount of information to take in.
In addition to knowing the opposing team, a catcher must have a great knowledge of his own pitchers and defense as well. He needs to be well-versed in what pitches his hurler is capable of, and even be able to instinctually know exactly what a pitcher is capable of on any given day. He needs to know when to give in to his pitchers wishes and when to stand his ground.
A catcher must also know exactly what is going on with his own defense. Where they are positioned and the range of each player is important when determining just what pitches a batter will see. A truly great catcher will often know where the play will take place before it even happens.
And if there's a runner on base? A catcher must make sure to hold him at bay and predict his actions while keeping track of the batter at the plate.