Most Major League starting pitchers have between three and four different kinds of pitches that they can throw throughout the game. Examples are
- Cut Fastball
The break and speed of these pitches vary quite dramatically so it's important for the catcher to know what's coming.
The reason that a pitcher has so many pitches is so that he can vary the speed, rotation, location and break to the batter. If the batter knows what pitch is coming they can largely negate the advantage that the pitcher creates by being able to mix their pitches.
Generally you want signs to be simple between a pitcher and a catcher. This is due to the fact that miscommunication with signals can lead to passed balls, wild pitches and even injury to the catcher.
This means that most of the time it makes sense for the catcher and pitcher to communicate via a simple system. The catcher shows the sign between his legs and the pitcher either nods to accept the sign or shakes his head to tell the catcher to change pitches. Then the catcher places his glove in the intended location and that becomes the target location for the pitch.
However, this is not the whole story. The catcher is generally not just calling pitches. Generally pitch selection is either the result of prior planning and discussion with the pitcher (they will generally meet before a game to work on how to get specific hitters out). Or the pitches are relayed by the manager or pitching coach from the dugout to the catcher and then to the pitcher.
Some specific situations where this system changes:
- When a runner reaches second base the catcher will almost always go to the mound to tell the pitcher that he will be switching to a more complicated signally system so that the runner (who has full view of his signalling) has a harder time determining the signs and cannot relay information to the batter)
- In late and close games with runners on base you might see the catcher go to the mound every pitch or every couple of pitches. This is because the margin for error is very slim and they want to avoid any potential confusion due to signals getting crossed.
This mostly applies to starting pitchers. When a reliever enters the game usually they throw fewer pitches (some great ones only have a single pitch, see Mariano Rivera with the cut fastball), and they will likely not have as well developed of a gameplan with their catcher. This is where having a veteran catcher who knows a pitcher very well is really important. He potentially knows the hitters better than the pitcher does and is the guide for the reliever to get outs.