Confusion arises from indiscriminate use of "scale" and "gauge" synonymously. The word "scale" strictly refers to the proportional size of the model, while "gauge" strictly applies to the measurement between the inside faces of the rails. It is completely incorrect to refer to the mainstream scales as "HO gauge", "N gauge, "Z gauge", etc. This is further complicated by the fact some scales use several different gauges; for example, HO scale uses 16.5 mm as the standard gauge of 4 ft 8 + 1 ⁄ 2 in (1,435 mm), 12 mm to represent 1,000 mm (3 ft 3 + 3 ⁄ 8 in) gauge (HOm), and 3 ft 6 in (1,067 mm) (HOn3-1/2), and 9 mm to represent a prototype gauge of 2 ft (610 mm).