The International Rule (also known as the Metre Rule), ratified in 1907, enabled fair competition between yachts by establishing a rating formula that allowed leeway for experimentation while minimizing huge disparities between boat designs. “The Rule” requires designers and builders to factor a combination of specific weighted measurements including sail area, waterline length and girth into a formula whose sum is no greater than the numerical class designation, for example 6 metres. Designers may change any of the formula variables and other details including keel and rudder shapes and sizes, provided that the final calculation is within the designated metre size and the resulting boat is sea-worthy. By utilizing the International Rule to create class rated boats, the resulting design innovation contest becomes an integral element of the ultimate competition. In addition to seamanship and tactics, Cup winners were often determined by who had built the fastest sailing “mousetrap” to round the course on race day.