Double-0 scale model railways were launched by Bing in 1921 as 'The Table Railway', running on 16.5 mm track and scaled at 4 mm to the foot. In 1922, the first models of British prototypes appeared. Initially all locomotives were powered by clockwork, but the first electric power appeared in autumn 1923.
OO scale uses the same track gauge as H0 gauge (3.5 mm to the foot, 1:87). However, the large propulsion mechanisms could not fit into the small British prototypes, so the scale was enlarged to 4 mm to the foot without altering the gauge. This means that the scale gauge represents 4'1½", seven inches narrower than the prototype 4'8½". It is also used to represent the 5' 3" Irish gauge, where it is a scale 13½ inches too narrow. These noticeable differences are aggravated by the over-scale rail section, over-scale wheel width and very deep wheel flange. These departures from scale require much larger clearances on pointwork and are particularly noticeable when looking down the track.
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