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HO scale a reduction of size of 1:87.1 in the model which is equal to 3.5 mm to one foot. HO standard gauge uses a track gauge of 16.5 mm (.649"). When the scale for HO was set many years ago, O scale models were built to a scale of 7 mm to the foot. So HO was "Half O" or 3.5 mm to the foot. [excerpt from nmra.org]