Grand Slam
A home run with the bases loaded, worth four runs, and the largest single swing in the game.
What it actually is. A grand slam is a home run hit with runners on first, second and third. All four men score. It is the maximum number of runs a single plate appearance can produce, since there is no way to have more than three men on base.
What somebody who knows the game gets out of it. The reason a grand slam feels enormous is that it usually arrives in a situation that was already enormous. Bases loaded means the pitcher has lost the strike zone or the defense has failed twice, and win probability was already moving. As an event it is worth about four runs but as a swing in the odds of winning a game it is frequently worth more, because the bases loaded state it erases was itself worth over a run and a half of expectation on its own.
The numbers worth knowing. Around a hundred to a hundred and fifty grand slams are hit across the league in a season, out of five thousand home runs. The career record is 25.
What people get wrong about it. The pitcher is not charged four earned runs automatically. If any of the three runners reached on an error, those runs are unearned, so a grand slam can be worth four runs on the scoreboard and one on the pitcher's ERA. This is the most common place people notice the earned run distinction.
At Little League and travel ball. Bases loaded happens far more often in youth baseball, because walks stack, so grand slams are commoner than the professional rate suggests. The coaching point is on the pitching side: a bases loaded count at 11U is almost always the product of three balls thrown to a previous hitter, not of three hits.
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