Just to complicate things a bit, there used to be a domestic OO (Double-Oh) scale at 1:72. It rose in the late '30's and was popular through the '40's. Lionel made a line of OO models that were very nice for the period. The track gauge was 0.75", and the wheel and flangeway standards were the same as for HO. Some enterprising early O scale modelers noted that the track gauge was three feet in O scale, and began using OO mechanisms, wheels, trucks and track to build On3 models. If you look at an NMRA standards gauge for On3, it is actually labeled OO/On3.