Tonight three legends of swimming—Michael Phelps, Chad Le Clos, and László Cseh—turned in identical times to share silver in the 100m butterfly. Last night, Simone Manuel tied for gold with Canadian Penny Oleksiak in the 100m freestyle. Modern timing systems are capable of measuring down to the millionth of a second—so why doesn’t FINA, the world swimming governing body, increase its timing precision by adding thousandths-of-seconds?

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As it turns out, FINA used to. In 1972, Sweden’s Gunnar Larsson beat American Tim McKee in the 400m individual medley by 0.002 seconds. That finish led the governing body to eliminate timing by a significant digit. But why?

In a 50 meter Olympic pool, at the current men’s world record 50m pace, a thousandth-of-a-second constitutes 2.39 millimeters of travel. FINA pool dimension regulations allow a tolerance of 3 centimeters in each lane, more than ten times that amount. Could you time swimmers to a thousandth-of-a-second? Sure, but you couldn’t guarantee the winning swimmer didn’t have a thousandth-of-a-second-shorter course to swim. (Attempting to construct a concrete pool to any tighter a tolerance is nearly impossible; the effective length of a pool can change depending on the ambient temperature, the water temperature, and even whether or not there are people in the pool itself.)

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Sports that subject athletes to an identical course—bobsled, for example—can use thousandths because this question doesn’t matter. Speed skating uses thousandths, though given how start commands are issued in that sport and the incredibly slow speed of sound, maybe they shouldn’t.