Paris in October 1907 and ratified the first International rule which defined not only the rating of yachts and measurement rules but also construction regulations (scantlings) and racing rules. Delegates from this meeting went on to form the International Yacht Racing Union (IYRU); the precursor to the present International Sailing Federation (ISAF). The USA sent observers to the meetings but did not initially adopt the first International rule and continued with the Universal Rule based on the formula developed by Nathanael Herreshoff. Evolution of the rule The International rule eventually became the backbone of yacht racing. The Rule created a formula that took into account most problematic areas that had previously caused dissent among the racing nations. It does not restrict size—many individual classes were created. It allowed designers a degree of latitude—yet controlled unsafe extremes. It laid down construction rules and governed the use of materials—yet understood that the Rule must develop. First International rule Used from 1907–1920 where = waterline length (LWL) = beam = chain girth = difference between skin girth and chain girth = sail area = freeboard Second International rule Used from 1920–1933. where = waterline length (LWL) = chain girth = difference between skin girth and chain girth = sail area = freeboard Third International rule Used from 1933–1939. where = waterline length (LWL) = difference between skin girth and chain girth = sail area = freeboard Rule forms It is important to note the term Metre does not refer to the length of the yacht; it is the product of a formula and denotes the class. A Six Metre yacht can be from 10 metres to 12 metres in length. Metre rule has proven to be successful and enduring, seeing only minor revisions over the years. Whilst many different Metre Class yachts were constructed during those first heady years, three sizes have maintained their popularity; the 6, 8 and 12 Metre Classes. The year after the ratification, the 12 Metre Class were chosen for the Olympics—the 6 and 8 Metres were also used at different stages. The 12 Metre Class was used for the America's Cup events until 1987. A relatively new arrival is the one-person 2.4 Metre Class, currently active as a Paralympics class. International rule should not be confused with Square Metre Rule. Why "Metre" is in the names International rule yachts It is commonly believed that the term Metre refers