Under the older rule described by the USGA, the interfering ball could be lifted if the two balls lay within six inches of each other. With more space between them, the player had to negotiate the obstruction. In 1938 the USGA also allowed an interfering ball within six inches of the hole to be lifted; The R&A did not make that change.12
A scorecard from the 1934 U.S. Women's Amateur carried a six-inch measure for checking that distance. It is a small piece of playing equipment that helps explain a scene in older match reports: the line to the hole could include the opponent's ball as well as the contours of the green.12
The stymie disappeared from the rules in 1952, when the first joint USGA and R&A code took effect. That date marks the worldwide removal of the rule, following earlier differences between the two authorities.12
Today, Rule 15.3b lets a player require another ball to be marked and lifted when they reasonably believe it might interfere with their play. An interfering ball-marker can also be moved under Rule 15.3c, then returned by reversing the measurement. A ball blocking a putt no longer creates the old stymie challenge.3