Double
Two bases on one swing, and the cleanest single sign that a hitter drives the ball.
What it actually is. A double is a hit on which the batter reaches second base. Most are line drives into a gap or balls off an outfield wall. A ground rule double is different: a fair ball that bounces over the fence or becomes lodged in it, on which every runner is awarded two bases regardless of where they would have got to.
What somebody who knows the game gets out of it. Doubles are the most honest counting stat a hitter has, because they are hard to get lucky into. Home runs depend heavily on park and on pull side, singles depend heavily on where the defense stood, but a double usually means the ball was hit hard into space, and doubles per plate appearance ages the way real skills age. Isolated power, slugging minus average, exists to separate this from singles, and it is one of the better simple reads on a hitter's contact quality.
The numbers worth knowing. A good doubles hitter posts 35 to 45 in a season; 50 is exceptional. The league hits somewhere around 1.6 doubles per team game. A double is worth about .77 runs by linear weights.
What people get wrong about it. A ground rule double can cost a run. If a man on first would have scored on a ball that bounced into the stands, he does not: he is awarded two bases and stops at third. The rule is a limit, not a gift, and the hitting team is frequently the one it hurts.
At Little League and travel ball. On a youth field with a short outfield fence and no wall to play, most extra base hits are doubles or triples produced by an outfielder misplaying a ball rather than by the ball beating anybody. The instructive number for a youth hitter is not doubles but hard hit balls, which is why the coaching versions of these tools track contact rather than the scorebook.
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