How do boxers measure their reach?

middle knuckle to middle knuckle? middle finger to middle finger? Are the palms facing sideways or down?:think

 

With a tape measure.

 

Traditionally, they measure fingertip to fingertip, then add around ten inches to the total.

Randy Gordon had a better idea, measuring the length of a jab from the shoulder to the end of the arm. (He probably had the boxer being measured clench the fist.) Gordon's concept did not have historical context, so it never really caught on.

Cleveland Williams had obviously much longer arms than Liston, but Sonny had much broader shoulders, meaning he had a long collarbone. Ali also had longer arms than Liston, but again, nobody had broader shoulders.