I agree with PF, the diameter of a circle is starkly visually defined by it crashing into, but not beyond, the circumference. A radius requires the further action of discovering and using the centre of the circle.

This is like someone claiming the half inch is a better general unit of measure because there would be a smaller consistent rounding error. If this were the case, then 100/254th of an inch would be better. We could call it a centimeter.

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