HO scale model of Thomas the Tank Engine at B&O Museum

The name HO is derived from the fact that its 1:87 scale is approximately half that of 0 scale which was the smallest of the series of older and larger 0, 1, 2 and 3 scales introduced by Märklin around 1900. In most English-speaking markets it is pronounced "aitch-oh" and written with the letters HO.