HO scale

HO or H0 is a rail transport modelling scale using a 1:87 scale. It is the most popular scale of model railway in the world. The name H0 comes from 1:87 scale being half that of 0 scale. […] Rather than referring to the scale as "half-zero" or "H-zero", English-speakers have consistently pronounced it /eɪtʃ oʊ/ and have generally written it with the letters HO.

— Wikipedia contributors, "HO scale", Wikipedia, The Free Encyclopedia.