The Six Metre Rule 

The International Six Metre Class is a class of classic racing yachts. Sixes are a construction class, meaning that the boats are not identical but are all designed to meet specific measurement formula, in this case International Rule. At their heyday, Sixes were the most important international yacht racing class, and they are still actively raced around the world. "Six metre" in class name does not, somewhat confusingly, refer to length of the boat, but product of the formula; 6mR boats are, on average, 10–11 metres long.