First base, or 1B, is the first of four stations on a baseball diamond which must be touched in succession by a baserunner to score a run for that player's team. A first baseman is the player on the team playing defense who fields the area nearest first base, and is responsible for the majority of plays made at that base. In the numbering system used to record defensive plays, the first baseman is assigned the number 3.
Also called first sacker or cornerman, the first baseman is ideally a tall player who throws left-handed and possesses good flexibility and quick reflexes. Flexibility is needed because the first baseman receives throws from the other infielders, the catcher and the pitcher after they have fielded ground balls. In order for the runner to be called out, the first baseman must be able to stretch towards the throw and catch it before the runner reaches first base. First base is often referred to as "the other hot corner"—the "hot corner" being third base—and therefore, like the third baseman, he must have quick reflexes to field the hardest hit balls down the foul line, mainly by left-handed pull hitters and right-handed hitters hitting to the opposite field. They often are power hitters who have a substantial number of home runs and extra base hits while maintaining a .270 plus batting average.
Good defensive first basemen, according to Bill James, are capable of playing off first base so that they can field ground balls hit to the fair side of first base. The first baseman then relies upon the pitcher to cover first base to receive the ball to complete the out. Indications of a good defensive first baseman include a large number of assists and a low number of throwing errors by other infielders.
The nature of play at first base often requires first basemen to stay close to the bag to hold runners or to reach the bag before the batter. First basemen are not typically expected to have the range required of a third baseman, shortstop, second baseman or an outfielder. As a result, first base is not usually perceived to be as physically demanding as other positions. However, it can also be a very hard position to play; a large amount of concentration and timing is required. Though many play at first base their entire career, it is common for veteran players to be moved to first base to extend their careers or to accommodate other recently acquired players. Facing a possible trade or a considerable reduction in playing time, a player will usually opt to move to first base instead. Catchers and corner outfielders are often moved to first base due to deteriorating health or if their fielding abilities at their original position are detrimental to the team.
Unlike the pitcher and catcher, who 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) the first baseman and the other fielders can vary their positioning in response to what they anticipate will be the actions of the batter and runner(s) once play begins.
When first base is not occupied by a baserunner, the first baseman usually stands behind first base and off the foul line. The distance he plays from the base and foul line is dependent on the current hitter and any runners on base. The exact position may also depend on the first baseman's experience, preference, and fielding ability. For a known right-handed pull hitter, the first baseman might position himself further towards the second baseman's normal fielding position. For a known left-handed pull hitter, the first baseman will position himself closer to the foul line to stop a ball hit down the line.
To protect against a bunt on the first base side of the infield, the first baseman will position himself in front of the base and move towards the hitter as the pitch is thrown. As soon as the pitcher commits to throwing towards home plate, the first basemen will charge towards the hitter to field the bunt. During these plays, it is the responsibility of the second basemen to cover first base.