Tag up
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In baseball, to tag up is to retouch or remain on the runner's time-of-pitch base until (after) the ball is first touched by a fielder. A runner must, by rule, tag up only when a batted ball is caught in flight. After a legal tag up, even if the ball was caught in foul territory, runners are free to attempt to advance. On long fly ball outs, runners can often gain a base; when a runner scores by these means, this is called a sacrifice fly. On short fly balls, runners seldom attempt to advance after tagging up.
After a caught fly ball, if a fielder with the ball can touch the runner or the runner's time-of-pitch base before he tags up, the runner is out. This often occurs on infield line drives.
A fielder may also appeal to an umpire that the runner left his time-of-pitch base too early. If the umpire agrees that the runner did not tag up, the umpire will call the runner out, regardless of what the runner had done after the ball is in play.
