The catcher has a position behind home plate where he can catch the pitches of the pitcher that the batter does not hit or (rarely) hit the batter and to throw the ball back to the pitcher. Such is the most basic play of the catcher, and it is most of his plays. The catcher has a position too dangerous to not have protective gear. In addition to the uniform that other players have the catcher as a rule always wears a batting helmet, a mask, a chest protector, and a groin guard to protect against errant pitches, batter's swings, and foul balls hit off him.His gear is similar to that of the home-plate umpire who faces much the same hazards. He has shin guards to protect him against a base runner trying to score on a play which may be the difference between an out and a run. The runner can be charging him at full running speed or in a dive known as a slide. He wears a unique glove for catching the many pitched balls.
The catcher goes into a crouch to catch pitched balls. Such is eventually difficult upon the catcher's knees, but it is necessary because the balls thrown through or near the strike zone can be caught reliably only in a crouch.
The catcher must, as mentioned above, catch or constrain any pitched balls and throw them back to the pitcher. Failure to catch or constrain such pitches with runners on base can result in baserunners advancing. Such uncaught or unconstrained pitches may be called passed balls if the scorer considers the fault with the catcher or wild pitches if the pitcher throws an unusually difficult pitch to catch, but the effect is much the same. Throwing the caught or constrained pitch back to the pitcher is routine, but any badly-thrown ball by the catcher is potentially an error should any baserunner advance.
On a foul tip with two strikes on the batter, the batter is out with a strikeout if the catcher can catch the foul tip before it hits the ground.
On a called or swinging third strike with fewer than two out and no runner on first base the catcher who does not catch the ball must gather it and throw it to first base, where the fielder at first base (usually the first baseman) must catch the ball and touch first base (usually with his feet) or, rarely, tag the batter running to first base to complete the out. Failure to complete this task results in a baserunner reaching first base without making an out. This is usually a routine play -- unless the pitcher has made a very bad pitch. (The batter is automatically out if there is a called or swinging third strike with fewer than two outs, but baserunners can attempt to advance on any called or swinging third strike. With the bases loaded and two outs on a called or swinging third strike, the catcher can gather the ball and perhaps touch home plate for a force of the baserunner from third base to end the inning.
On efforts by the offensive team to steal second or third base the catcher can throw to a fielder at or near the relevant base who at best (for the defensive team) tags the baserunner for an out instead of an advance of a base. Much of the defensive reputation of a catcher depends upon his ability to turn stolen base attempts into outs or to at least deter stolen bases.
Catchers never catch line drives -- but they often catch high pop flies near home plate, many of them foul. The catcher is the only fielder who has any chance of catching such foul fly balls. Those fly balls often take unusual angles and may require quick responses. Mobile catchers catch more of them. Catchers also field weakly-hit fair ground balls (especially bunts)that do not go far from home plate. The catcher sometimes must make a swift decision on whether fielding them fair and throwing them to a base to get an out or to let them go foul with at most an additional strike, and if he does field the ball with runners on base, to which base to throw the ball.
Catchers take the role of basemen when an out can be made at home plate on a fielder's choice play. With the bases loaded and a ball hit to another infielder or the pitcher, the infielder or pitcher might throw to the catcher at home plate to get an out should the catcher tag home plate with a part of himself (usually a foot) or the ball in or not in the glove. If the bases are not loaded, there might still be a play at the plate, but this is not a force play. The catcher must then tag the baserunner who tries to score. Running at full speed the baserunner may try to slide around the catcher and evade the tag or slam in to the catcher and try to dislodge the ball. A catcher who stops the slide or holds the ball when he is slammed into turns what might other be a run for the opposing team into an out. Note well: a baserunner trying to score is usually running at top speed.
The catcher often makes signals to the pitcher to the pitcher to throw certain pitches as strategy for fooling the batter. The catcher has another, less visible responsibility: to be aware of the ability of the pitcher to pitch competently. As the game progresses a pitcher may become more 'wild' (failing to get strikes or throwing balls in areas in which they are easily hit hard)-- or the pitcher's signature pitches could become slower or flatter and easily be hit. The fastball that loses its velocity, the curve ball that doesn't keep its curve, the breaking pitch that does not break, the slider that does not slide, or the sinkerball that does not sink begins to look like a batting-practice fastball that can be hit easily, hard, or far. The catcher may have to tell the manager or pitching coach that a pitcher is thus losing his ability to pitch competently.
On rare occasions a catcher may run to a different position and participate as if an infielder in rundown plays.