The fourth out might sound like a mere bone for rulebook lawyers to pick, but failing to grasp its ins and outs can cost games.
Take the Arizona Diamondbacks in their April 12, 2009, outing against Los Angeles. In the second inning, Dodger Randy Wolf comes to bat with one out and runners Juan Pierre on second and Andre Ethier on third. Wolf drills a line drive to pitcher Dan Haren, who makes the catch, then chucks the cowhide to Felipe Lopez at second. With Wolf's ball caught, Pierre and Ethier, who ran at the crack of the bat, must now retouch second and third, but Ethier continues home, crossing the plate before Pierre is tagged out [sources: Gurnick; Hernandez; MLB; Piecoro].
Ethier's failure to tag up should trigger a fourth out that undoes his run, but the D-backs leave the field without making the necessary appeal play. Plate umpire Larry Vanover rules that the run is good, granting Los Angeles the tying run in what would be a 3-1 victory over Arizona. Too bad Lopez didn't just touch second base instead of running down Pierre and giving Ethier time to score. It would have saved a lot of trouble [sources: Gurnick; Hernandez; MLB; Piecoro].
Getting the hang of it? Let's try a more theoretical example, this time from the June 10, 2010, match between the Minnesota Twins and the Kansas City Royals. It's the bottom of the third, there's one out, and the Twins trail by three. Minnesota's Joe Mauer steps to the plate with runners Denard Span on second and Nick Punto on third. Mauer flies out, and Punto and Span scramble to tag up, but Span is beat out at second and the Twins are retired [sources: Boeck; MLB].
Rewinding the tape, we see that Span never touched third while returning to second -- a moot fourth out, since no run was at stake. But suppose Punto had made it home before Span's out? Better still, what if Span had been safe at second (but still cut third base) and Punto had scored (but failed to tag up)?
Confusing? Just wait. As Scott Boeck of USA Today points out, under these circumstances, the Royals could make two appeal plays -- one against Span for failing to touch third and one against Punto for not tagging up after the fly out. But which appeal would make the third out, and which would become the fourth out that replaced it [sources: Boeck; MLB]?
Answer: Whichever one Kansas City chose.
According to rule 7.10, when the defense makes more than one appeal during a play that would end a half-inning, it can choose which third out to keep. In this case, the Royals would keep Punto's fourth out to nix his run [sources: Boeck; MLB].
It's just that simple.