Double Play Depth
Middle infielders shade toward second to turn two.
How it actually works. Ordinary depth is about range. Double play depth trades some of that range for time: it puts the shortstop and second baseman close enough to second that whichever one is not fielding the ball can be standing on the bag, set and waiting, when the throw arrives. The feed, the turn and the throw all have to happen inside about four seconds from contact, so half a step of positioning is decisive.
Why it is done, and when it is wrong. The double play is the single most valuable defensive event in baseball, because it takes two outs and usually removes a runner in scoring position in one action. That is why a defense will accept a real cost in range to make it available, and why it is set with a runner on first and fewer than two out and nowhere else. Turning two is also the reason the middle infield is the hardest defensive assignment on the field: the pivot man has to catch, transfer, throw across his body and get out of the way of a runner, all in about half a second.
What people get wrong about it. Ground ball rate is the whole bet. Setting up for two against a hitter who lifts everything is giving away range for a play that will not come, and good defensive coordinators do not do it just because the base is occupied.
At Little League and travel ball. The double play is rare in youth baseball and the depth is usually the wrong call, because the turn requires a feed and a pivot that most players below the mid teens cannot execute. The right instruction is almost always to take the sure out.
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