The first baseman has an important defensive role as the infielder most likely to finish an infield out on any ground ball. Usually located very close to first base, he is of course in position to catch any fly balls or line drives hit in or near his direction. He also fields ground balls hit toward him, and usually has a short run to first base so that he can reach first base with the fielded ball before the batter can reach first base by running to first base. In a rare play, the first baseman may receive a throw from a catcher on a dropped called or swinging third strike; if the first baseman catches the ball and reaches first base to tag the bag before the batter can reach first base, then the batter is out on the strikeout.
Under some circumstances he may play far enough off first base so that he must make a toss of a ground ball to the pitcher who typically has a shorter run to first base than does the batter. Such is common with a batter who tends to hit balls between first and second base. With a bung a first baseman may need to charge the bunted ball and throw to a base where a play is possible. When the person to be put out is the batter, it may be the second baseman who runs from the first-base side of second base to reach first base before the batter.
The first baseman rarely has a long run on any play except for a foul pop fly. High pop flies often give him plenty of time to situate himself under the pop fly to be caught for an out. A baserunner can try to run to the next base after the first baseman catches the ball, so in such a situation the first baseman might benefit from a strong throw to another base or to the catcher at home plate to prevent the advance of a baserunner. With a runner on first base, the first baseman may situate himself to receive a throw from the pitcher intended to catch a baserunner who strays too far from first base (this play is called a pick-off) or, should the runner try to steal second base, get a throw from the pitcher and throw to the fielder (usually a second baseman or shortstop) guarding second base in an attempt to get the runner put out in a play known as "caught stealing". The first baseman may be part of a rundown play.
On another rare play, a first baseman may be the one to catch a thrown ball from another fielder when a baserunner from first base leaves first base on a caught fly ball (including a line drive). The baserunner must return to first base before the first baseman can tag first base, lest the batter be out.
Good defensive first baseman can retrieve inaccurately-thrown balls from other fielders; such prevents errors that would allow batter-baserunners to reach first base safely with much the same effect as a hit.
In major-league play, the first baseman is typically one of the slowest runners holding a defensive position. Speed is less critical to the first baseman than to any fielders other than the pitcher (who has far more responsibility for pitching than for chasing down ground balls or retrieving infield flies) or the catcher. A first baseman has often been moved from some other position in which he has become a defensive liability due to a loss of strength of his throwing arm or of speed. Very good hitters are often moved to first base toward the ends of their careers. It is also possible that a team may move a good hitter from a position in which his potential for injury is high. As a general rule, first basemen are typically among the best pure hitters on any major-league team.
In scoring a game, the first baseman is indicated with the number "3". Thus an unassisted out played by the first baseman is scored "3" ("3F" if foul), a ground-out, shortstop to first is "63", and a ground-out, first baseman to pitcher is "31".