Once a batter becomes a runner and reaches first base safely, he is
said to be "on" that base until he attempts to advance to the next base,
until he is put out, or until the half-inning ends. In order to be safe
a runner must beat the ball to the bag. When two or more runners are on
the basepaths, the runner farther along is called a lead runner or a preceding runner; any other runner is called a trailing runner or a following runner. Runners on second or third base are considered to be in scoring position since ordinary hits, even singles, will often allow them to score.
A runner legally touching a base is "safe" — in most situations he may not be put out. Runners may attempt to advance from base to base at any time (except when the ball is dead), but must attempt to advance when forced when all previous bases are occupied and a batted ball that touches the ground is a fair ball. When a batted ball is hit in the air, i.e., a fly ball, and caught by the defending team, runners must return and "retouch" the base they occupied at the time of the pitch. The common name for this requirement is tagging up. If the runner retouches the origin base at any time after the fly ball is first touched by a fielder, he may attempt to advance to the next base or bases at his own risk. The penalty for failing to retouch (if the defensive team notices this) is that the advancing runner can be put out on a live appeal in which the defensive team player with the ball touches the base from which that runner departed prematurely, a process that looks like a force play to the eye, but technically is not because runners are not "forced" backwards to their origin bases. If a runner tries to run the next base in sequence, they can be deemed out by either of two methods. The runner may be tagged by the infielder, or the runner may fail to reach the base before the baseball has arrived at it. However, if the runner is not forced to run to the next base in sequence, they are not deemed out until they are tagged. The runner, however, cannot take a base that is occupied by a fieldman holding the baseball.
Only one runner may occupy a base at a time; if two runners are touching a base at once, the trailing runner is in jeopardy and will be out if tagged. However, if the trail runner reached the base having been forced there, it is the lead runner who will be out when tagged for failing to reach his force base. Either such occurrence is very rare. Thus, after a play, at most three runners may be on the basepaths, one on each base—first, second, and third. When three runners are on base, this is called bases loaded.
Baserunners may attempt to advance, or steal a base,
while the pitcher is preparing to make a pitch, while he is making a
pitch, or while waiting for a return throw from the catcher after a
pitch. The pitcher, in lieu of delivering the pitch, may try to prevent
this by throwing the ball to one of the infielders in order to tag the
runner; if successful, it is called a pick-off. He may also, as
part of a planned sequence, throw a pitch well outside and high of the
strike zone to his catcher who is waiting for it upright there, and is
thus better prepared to throw out a runner trying to steal; this
sequence is called a "pitchout." If the runner attempts to steal the
next base but is tagged out before reaching it safely, he is caught stealing. An illegal attempt by the pitcher to deceive a runner, among other pitching violations, is called a balk, allowing all runners to advance one base without risk of being put out.
Another fundamental tenet of the rules of baseball is that a runner who was initially ruled out can subsequently be ruled safe, but once a runner is ruled safe he cannot be called out on the same play, unless he overruns the base. For example, if a baserunner steals second base, beating the throw, an umpire might make the quick call of safe, but if the runner then slides beyond the base and is tagged before he can retreat to it the umpire has the right to change the call. A runner initially called out can be subsequently ruled safe if the fielder putting the runner out drops the ball (on either a tag or force play), pulls his foot off the base (in the case of a force play), or otherwise illegally obstructs a runner from reaching a base that he otherwise would have reached safely.
A runner legally touching a base is "safe" — in most situations he may not be put out. Runners may attempt to advance from base to base at any time (except when the ball is dead), but must attempt to advance when forced when all previous bases are occupied and a batted ball that touches the ground is a fair ball. When a batted ball is hit in the air, i.e., a fly ball, and caught by the defending team, runners must return and "retouch" the base they occupied at the time of the pitch. The common name for this requirement is tagging up. If the runner retouches the origin base at any time after the fly ball is first touched by a fielder, he may attempt to advance to the next base or bases at his own risk. The penalty for failing to retouch (if the defensive team notices this) is that the advancing runner can be put out on a live appeal in which the defensive team player with the ball touches the base from which that runner departed prematurely, a process that looks like a force play to the eye, but technically is not because runners are not "forced" backwards to their origin bases. If a runner tries to run the next base in sequence, they can be deemed out by either of two methods. The runner may be tagged by the infielder, or the runner may fail to reach the base before the baseball has arrived at it. However, if the runner is not forced to run to the next base in sequence, they are not deemed out until they are tagged. The runner, however, cannot take a base that is occupied by a fieldman holding the baseball.
Only one runner may occupy a base at a time; if two runners are touching a base at once, the trailing runner is in jeopardy and will be out if tagged. However, if the trail runner reached the base having been forced there, it is the lead runner who will be out when tagged for failing to reach his force base. Either such occurrence is very rare. Thus, after a play, at most three runners may be on the basepaths, one on each base—first, second, and third. When three runners are on base, this is called bases loaded.
Another fundamental tenet of the rules of baseball is that a runner who was initially ruled out can subsequently be ruled safe, but once a runner is ruled safe he cannot be called out on the same play, unless he overruns the base. For example, if a baserunner steals second base, beating the throw, an umpire might make the quick call of safe, but if the runner then slides beyond the base and is tagged before he can retreat to it the umpire has the right to change the call. A runner initially called out can be subsequently ruled safe if the fielder putting the runner out drops the ball (on either a tag or force play), pulls his foot off the base (in the case of a force play), or otherwise illegally obstructs a runner from reaching a base that he otherwise would have reached safely.