In baseball, the pitcher is the player who throws the baseball from the pitcher's mound toward the catcher to begin each play, with the goal of retiring a batter, who attempts to either make contact with the pitched ball or draw a walk. In the numbering system used to record defensive plays, the pitcher is assigned the number 1. Traditionally, the pitcher also bats.
Rotation and specialization
Pitching is physically demanding, especially if the pitcher is throwing with maximum effort. A full game usually involves 120–170 pitches thrown by each team, and most pitchers begin to tire before they reach this point. As a result, the pitcher who starts a game often will not be the one who finishes it, and he may not be recovered enough to pitch again for a few days. The act of throwing a baseball at high speed is very unnatural to the body and somewhat damaging to human muscles; thus pitchers are very susceptible to injuries, soreness, and general pain.
Teams have devised two strategies to address this problem: rotation and specialization. To accommodate playing nearly every day, a team will include a group of pitchers who start games and rotate between them, allowing each pitcher to rest for a few days between starts. A team's roster of starting pitchers are usually not even in terms of skill. The best starter in the team's rotation is called the ace.
After the ball is pitched
The pitcher's duty does not cease after he pitches the ball. Unlike the other fielders, a pitcher and catcher must start every play in a designated area. The pitcher must be on the pitcher's mound, with one foot in contact with the pitcher's rubber, and the catcher must be behind home plate in the catcher's box. Once the ball is in play, however, the pitcher and catcher, like the other fielders, can respond to any part of the field necessary to make or assist in a defensive play.
At that point, the pitcher has several standard roles. The pitcher must attempt to field any balls coming up the middle. He must head over to first base, to be available to cover it, on balls hit to the right side, since the first baseman might be fielding them. On passed balls and wild pitches, he covers home-plate when there are runners on. Also, he generally backs up throws to home plate. When there is a throw from the outfield to third base, he has to back up the play to third base as well.
Pitching in a game
A skilled pitcher often throws a variety of different pitches to prevent the batter from hitting the ball well. The most basic pitch is a fastball, where the pitcher throws the ball as hard as he can. Some pitchers are able to throw a fastball at a speed of over 100 miles per hour (161 km/h). Other common types of pitches are the curveball, slider, changeup, cutter, sinker, screwball, forkball, split-fingered fastball, slurve, and knuckleball.