OO

Double-O scale model railways were first launched by Bing in 1921 as 'The Table Railway', running on 16.5mm track and scaled at 4mm 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 the Autumn of 1923. In 1932, the Bing company collapsed, but the Table Railway continued to be manufactured by the new Trix company. However, Trix decided to use a new standard of 3.5mm to the foot or 1:87, and this scale came to be known as 'HO'. (It is thought that this may have originated as 'Half-O' gauge, but there is no proof of this.) In 1938, the Meccano Company launched a new range of OO (4mm to the foot) models under the trade name of Hornby DublO, and the OO scale has remained as the UK's most popular ever since.

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In computer science, OO stands for object-oriented.