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A font, with regard to the number on the back of uniforms, is said to be readable if ...
it can be read easily at a distance of 20 feet (6 meters)
if you stand two players on the same team next to each other, you can see there is a difference
it can be easily read and distinguished from the other players' numbers by the officials, other players, and fans.
it can be easily read and distinguished from the other players' numbers by other players