In baseball, to tag up is for a baserunner to retouch or remain on their starting base (the time-of-pitch base) until (after) the ball either lands in fair territory or is first touched by a fielder. By rule, baserunners must tag up when a fly ball is caught in flight by a fielder. After a legal tag up, runners are free to attempt to advance, even if the ball was caught in foul territory. 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, due to the high risk of being thrown out.