Fourth out

In baseball, the fourth out is a legal out made by the defense after three outs in a half-inning already have been made. According to the rules, the third out does not cause the ball to become dead; if the fielders make a subsequent out that prevents a run from scoring, this out will supersede the apparent third out, thus becoming the recorded third out. For statistical purposes, the apparent third out is "undone" and the fourth out's result is recorded instead. With the advent of video replay appeals, a new rationale for making extra out(s) has emerged - insurance against a prior out being undone on appeal. These fourth out situations are not the same as four strikeouts in an inning.

The motivation for making a fourth out is to nullify a scored run, by either putting out the runner who had scored (on appeal, if the player failed to tag up after a catch) or putting out an additional runner who is forced to advance.

No run may score on an inning-ending play in which the third out is a force out or on the batter before he reaches first base. Put in other words, force outs count before runs are scored. It is common that a runner reaches home plate a moment before the third out is made by force out. Such a case is routine; the runner doesn't score but is counted as left on base.

It is also common that the third out might come on a non-force tag out after another runner reaches home plate. By extension of these two rules, the "fourth out" covers the case where the third out is not a force out, but a subsequent out is. Since the force out counts before the run scores, it must also count before the third out.

Apparently there are no known MLB examples of a fourth out changing places with a prior out and thereby cancelling a run. The situations where a fourth out may be recognized are exceedingly rare, but some hypothetical examples, and two real examples where the fourth out rule did come into play, are noted below:

Suppose three runners are on base with two outs, and the batter hits the ball within the field of play for an apparent hit. Two important facts are required:

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