H0 is a rail transport modelling scale using a 1:87 scale (3.5 mm to 1 foot). It is the most popular scale of model railway in the world. The rails are spaced 16.5 mm (0.650 in) apart for modelling 1,435 mm (4 ft 8 1⁄2 in) standard gauge tracks and trains in H0.
The name HO comes from 1:87 scale being half that of O scale, which was previously the smallest of the series of older and larger 0, 1, 2 and 3 gauges introduced by Märklin around 1900. In most English-speaking markets it is pronounced “aitch-oh” and written with the letters HO today, but in other markets remains written with the letter H and number 0 (zero), so in German it is pronounced as “hah-null”.