The baseball field, or "diamond", has two main parts, the infield and the outfield. The "infield" is where the four bases are. The "outfield" is beyond the bases, from the view of home plate. The lines from home plate to first base and home plate to third base are the "foul lines", and the ground outside of these lines is called "foul territory". A ball that is hit with a bat and flies between the foul lines is a "fair ball", and the batter and runners can try and run around the bases and score. A ball that is outside the foul lines is a "foul ball". If the ball hits the ground in the foul area rather than being caught in the air, the batter continues to bat, and any runners must return to the base that they were on before the ball was hit. If the batter has fewer than two strikes, a foul ball counts as a strike. If the batter already has two strikes, and the foul ball is not caught in the air, then the batter continues to hit. If a ball is caught by a fielder in fair or foul ground, the batter is out.
The most important part of the game is between the "pitcher" and the "batter". The pitcher throws, or "pitches", the ball towards home plate. The pitcher normally throws the ball close enough for the batter to hit it. If the pitcher throws the ball in the strike zone, which is the area over home plate and between the hitter's knee and chest, the pitch is a "strike", unless the batter hits the ball. The pitch is always a strike, regardless of where it is, if the batter swings the bat and misses, so the batter must have good aim with the bat. Three strikes are a "strikeout", and this is one way to make an "out". A pitch that the batter does not swing at, and which is not called a strike, is a "ball." On the fourth "ball" thrown by a pitcher, the batter "walks" to first base, so it is important to pitch well.
The "catcher" for the pitcher's team waits behind the batter, and catches any ball that the batter does not hit. The catcher uses signals to tell the pitcher where to throw the ball. If the pitcher does not like what the catcher says, he will shake his head, which signals "no". If he agrees with what the catcher has signaled, he will nod his head, which signals "yes".
There are many ways to get batters out, and runners can also be gotten out. Some common ways to get batters out are catching a batted ball in the air, whether in fair or foul territory, throwing the ball to the defensive player at first base (an out if it gets there before the batter), and a strikeout. A runner can be put out by tagging the runner while the runner is not on a base, and by "forcing him out" (when a base is touched before a player can get there, with no base for the runner to go back to). When the fielding team has put out three of the batting team's players, the "half-inning" is over and the team in the field and the team at bat switch places.
The batting team wants to get runs. In order to get a run, a player must bat, then become a base runner, touch all the bases in order, and then touch home plate without being called out. So first, the batter wants to make other players get to home plate, or to run the bases himself. Runners can not pass each other while running the bases.
A base runner who touches home plate after touching all previous bases in order, and without getting out, scores a "run". If the batter hits the ball over the fence (between the foul lines) without touching the ground, it is a home run. The batter, and any base-runners, are allowed to advance to the home plate and score a run. The fielding team can do nothing to stop them.
The team on the field tries not to let the team who's batting get any runs. The fielding team has a pitcher and a catcher. The remaining seven fielders can stand anywhere in the field. However, there are usually four people that stand around the infield close to the bases and three "outfielders" who stand around the outfield.
The four infielders are the "first baseman", "second baseman", "shortstop", and "third baseman". The first baseman and third baseman stand close to first base and third base. The second baseman and the shortstop stand on either side of second base.
The first baseman's job is to make "force plays" at first base. In a force play, another infielder catches a ball that has touched the ground, and throws it to the first baseman. The first baseman must then touch the batter or the base with the ball before the batter can touch first base. Then the batter is out. First basemen need to have quick feet, stretch well, be quick and know how to catch wild throws. First base is one of the most important positions as a significant number of plays happen there.
The second baseman's job is to cover the area to the right of second base and to back the first baseman up. The shortstop's job is to cover the area between second and third bases. This is where right-handed batters usually hit ground balls. The shortstop also covers second or third base and the near part of left field. The shortstop is usually the best fielder on the team. The third baseman needs to have a strong throwing arm. This is because many times the batter will hit a ball toward third base. The third baseman must throw the ball very quickly to the first baseman, to get the runner out. Because the balls that go to third base are usually hit very hard, the third baseman must also be very quick.
The three outfielders are called the "left fielder", the "center fielder", and the "right fielder", because they stand in left field, center field and right field. Left field and right field are on the left and right sides, if you look out from home plate. Center field is straight ahead from home plate. Center field is very big, so the center fielder is usually the fastest.
The team can decide where to put the infielders and outfielders. Players often stand at slightly different places on the field between some plays. These changes are called "shifts". The fielders may shift at any time. Players can shift for many reasons. One of the more popular ones is the "defensive shift", where players move in the infield. They do this because they know that some batters can only hit a ball a certain direction. It can also be easier to make a double play when fielders are moved a certain way.
Teams can change pitchers during a game. Teams change their pitchers often because it is hard for a pitcher to throw a full game of nine innings. A pitcher can sometimes throw a no-hitter where no one on the opposite team gets an earned hit. A team can use as many pitchers as it wants to, but it is rare to use more than eight in a game. The ways that a pitcher throws the ball are called "pitches". Many professional pitchers use two or more different pitches. Pitchers change which pitch they throw so that the batter will be tricked and not know what pitch to expect. This makes it more difficult for the batter to hit the ball. Pitchers can make the ball move differently: faster or slower, closer or farther from the batter, higher or lower. There are also many types of pitches, such as the slurve, curve, slider, splitter, sinker, screw, 2-seam cut, 2-seam screw, knuckle, knuckle curve, change-up, circle change-up, palm ball, and others.
When throwing the ball, the pitcher must touch the pitchers mound with his foot. The pitcher's rubber is on top of the mound. The pitcher cannot take more than one step forward when he throws the ball. That makes the pitcher throw the ball slower. Many major-league pitchers can throw the ball up to 100 miles per hour (145 km/h). Throwing a baseball that fast can be bad for the body. Pitchers can end up with a lot of injuries. Doctors often will perform Tommy John surgery on a pitcher with an elbow injury. The operation is named after Tommy John, the first pitcher to have the surgery. Today, pitchers are able to recover from their injuries much more often than before Tommy John surgery.