The Bases
Ninety feet apart, and that one number is the reason the game is as close as it is.
What it actually is. Three canvas or rubber bags mark first, second and third, ninety feet apart on a square with home plate at one corner. Since 2023 each is eighteen inches square rather than fifteen. A runner must touch each in order, and first base is the only one he may overrun without being liable to be tagged.
What somebody who knows the game gets out of it. Ninety feet is the most quoted number in baseball design and it deserves the reputation: the routine ground ball to short beats the runner by a step or two, and every play at first is close. Nobody arrived at it by calculation, but the whole balance between offense and defense is sitting on it. When the bases were enlarged to eighteen inches, the distance between first and second fell by four and a half inches, which sounds trivial and moved the stolen base success rate by enough to change how the game is played.
The numbers worth knowing. Ninety feet between bases, 127 feet 3 inches from home to second. A major league runner covers ninety feet in roughly four seconds from a standing start and about 3.5 seconds home to first on a bunt.
What people get wrong about it. Only first base can be overrun. A runner who overruns second or third and is tagged is out even if he was safe a moment earlier, and a runner who overruns first and turns toward second can be tagged as well. That single asymmetry produces more free outs at every level than any other rule.
At Little League and travel ball. Youth base paths are 60 feet at Little League Majors and 65 to 70 feet in the middle travel ages before reaching 90. The change is more than proportional to the players: a 60 foot path with a 46 foot mound produces a game where stealing is nearly free unless the rules restrict leads, which is why Little League restricts them.
Batter's Eye: look up anything in baseball