In 1961, the International Yacht Racing Union established a Box Rule for four different multihull development classes. D, C, B, and A, which was the smallest and had the following basic dimension limitations: 18' overall length, 7'6" beam, 150 sq. ft. of sail area including mast. Since then, the class added a minimum weight requirement of 165 pounds to prevent the development of super-light, throwaway boats that could unnecessarily raise the cost of competing. The 165-pound minimum has worked out well, with many boats at this weight still competitive after many years of sailing.