H0 scale (H + number zero), often also written HO scale (H + letter O), is the most popular scale model|scale of model railway in most of the world (outside the United Kingdom, where the slightly larger 00 gauge is more common). The name is derived from the fact that its 1:87 scale is approximately half that of 0 scale|0 (zero) scale, hence H0. 0 scale in turn was named following the older and larger 1 gauge|1, 2 gauge|2, and 3 gauge|3 scales. The term H0 is pronounced ("aitch-oh"), not "ho" nor "aitch-zero".