HO-Scale 1:87

HO or H0 is a modeling 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 millimeters (0.650 in) apart.

The name HO comes from 1:87 scale being half that of O Scale.  This scale is written with the letters HO today, but in other markets remains written with the letter H and number 0 (zero)