Tagging up means a runner must be touching his base when a fly ball is caught before he can try to advance. He is allowed to leave the moment a fielder first touches the ball, not when the catch is secured. If a runner leaves early, the defense can appeal by throwing to that base, and he is called out. Tagging up is how runners score on a sacrifice fly.
Common Tag Up Situations
| Situation | What usually happens |
|---|---|
| Runner on third, fewer than two outs, deep fly | Runner tags and scores for a sacrifice fly |
| Runner on second, deep fly to right | Runner tags and takes third, since it is a long throw |
| Runner on first, routine fly | Runner holds; stealing second on a tag is rare |
| Line drive caught at an infielder | Runner has to scramble back or be doubled off |
| Two outs | No tagging; the catch ends the inning, so runners go on contact |
The Appeal Play
Umpires do not call a runner out for leaving early on their own. The defense has to appeal, usually by throwing the ball to the base the runner left or by tagging the runner, before the next pitch. That is why you sometimes see a pitcher step off and throw to third after a run scores on a fly ball.
Related
What is a sacrifice fly · Distance between bases · Baseball slang glossary
Rules summarized from the MLB official rules.