| Size |
 | | Ring gauges for testing ball diameter |
Diameter and mass are the oldest ball specifications, having been in the Rules of Tennis since 1880. Although it would appear to be an easy task, accurate and repeatable measurement of ball diameter is difficult. Whether a ball is of an appropriate size is determined using two ring gauges in a “go/no-go” manner. A ball must pass through the larger one under its own weight, but not through the smaller one.
Each ball is rotated on the gauges to check its concentricity. The internal diameters of the ring gauges are calibrated to 0.0005 cm! The internal diameters of the two rings are calibrated to the acceptable range for ball diameter (2.575 inches to 2.700 inches respectively for a Type 2 ball). The accuracy of calibration is 1/-0.005 inches. This specification last changed in 1966 (from 2.575-2.675 inches).
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