Dinghy designs are often referred to as "classes"; these classes are usually categorised as one design, open, or restricted. A more formal term for open is "Development Class". One design dinghies are supposed to be identical, though in reality this is not always the case. Only the most restrictive one design classes will restrict individual fittings. Then there becomes a sliding scale of allowable modification or design differences; restricted classes would typically allow the movement of fittings or even parts of the boat around, but are unlikely to allow major changes to hull shape or sails. Truly open development classes are also almost unknown, the famous line about the 18 ft skiff "the boat shall be 18ft long and the race starts at 2 o'clock" is a myth but open classes will usually allow pretty radical changes within usually some kind of box rule which specifies depth, length, width of hull height of mast and sometimes a minimum weight and sail area.