In baseball, a middle infielder is a player whose position is one of the two infield positions that are closest to second base — either shortstop or second baseman. The term "middle infielder" is based on the fact that second base is closest to the middle of the field; first base and third base are on the sides, next to the foul lines. The shortstop plays on one side of second base — the side closer to third base, or the left side when viewed from home plate — and the second baseman plays on the other side of second base. The first baseman and third baseman are called corner infielders.
Playing a middle infield position requires a great deal of athletic ability, especially agility and quickness. Middle infielders typically must be able to cover more ground than corner infielders, which means that they must be quick enough to run a greater distance in time to field a ball that has been hit. Either the second baseman or shortstop usually is responsible for catching a throw to second base, so they must maintain constant communication to determine which player will cover the base in the various situations that might occur. Which player covers the base typically depends on factors such as who is throwing the ball to second base; where the ball went, if it was hit; or whether a right-handed batter or left-handed batter is at the plate, if a runner at first base is trying to steal second and the throw is coming from the catcher.
One key aspect for a middle infielder is turning a double play. This occurs when a runner is on first base and the batter hits a ball to any of the infielders, the pitcher or the catcher. The infielders then coordinate to throw out both the runner heading toward second base and the batter running toward first base. When both players are thrown out successfully in the same play, this is called a double play.
The most common type of double play involves both middle infielder positions, as well as the first baseman, and is called a 6-4-3 double play, with the numbers indicating the player’s position on a scorecard. This type of play means that the shortstop fielded the ball, then flipped or threw it to the second baseman covering the base, and the second threw the ball to first base to record the second out. This type of double play requires communication and agility on the part of both middle infielders. Agility also is important, because the runner attempting to reach second base often will try to slide into and thereby disrupt the middle infielder who is trying to turn the double play. The middle infielder often must be able to throw the ball to first base while leaping out of the way of the sliding runner, in order to avoid a collision and possibly an injury.
Middle infielders often wear smaller gloves than most other players on the field, with second basemen typically wearing the smallest. Wearing a smaller glove helps the middle infielder be able to reach into the glove after catching the ball and take out the ball quickly to throw it. This is important after the infielder fields a batted ball, and it is crucial when he or she is trying to quickly turn a double play. Even a slight delay or fumble as the middle infielder tries to take the ball out of the glove could mean the difference between the batter being safe or out at first base.