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OO (letters, sometimes written as numbers, 00) is a hybrid gauge used by British modellers. Track gauge is as for HO scale, 16.5mm, which corresponds to 3.5mm to the foot, but the scale for construction of locomotives, rolling stock and scenery is approximately 4mm to the foot, 1/76.

Finescale modellers will convert OO RTR to one of the 4mm scale gauges such as EM4 or Proto-4.