smaller than 1.680 inches

The current regulations mandated by the R&A and the USGA state that diameter of the
golf ball cannot be any smaller than 1.680 inches. Until 1990, it was permissible to
use balls of LESS than 1.62 inches in diameter in tournaments under the jurisdiction
of the R&A. This ball was commonly called a "British" ball while the golf ball
approved by the USGA was simply the "American ball".
The maximum velocity of the ball may not exceed 250 feet per second (76 m/s) under
test conditions and the weight of the ball may not exceed 1.620 ounces.In those
earlier times, when rules were stretched to accommodate some fun in a friendly round
players would slip a small ball (British) into play for into the wind shots.