To add further confusion, H0, 00, O, N, Z or ZZ are not exactly "scales" either, they are gauges. Originally they were designating the track measurments, I believe.
Some are, it's true. HO is actually defined as 3.5mm to the foot (crazy way to do it, isn't it?), which is why I said "approximately 1/87 (it's actually 1:87.0857), although NMRA standards specifies the track gauge to be 16.5mm (to represent 56.5" standard gauge), making the ratio 1:86.9758). So, approximately 1/87. O gauge/scale is usually 1:48, and that is the scale most O gauge railway equipment is manufactured as, but Lionel and others have always used a 1.25" gauge, which is oversize (the same as Russia uses, I believe, representing 5' wide gauge). Thus, some fine scale modelers make their equipment 1:45.2, making the gauge standard. But you won't find any model structures and such in that scale.
The same thing happens with N gauge/scale. The 9mm track gauge ends up with a scale of 1:148 track scale in Great Britain, but US uses 1:160, and to my knowledge, all N gauge equipment is manufactured as 1:160 scale. Z scale is actually 1:220, or 1.385mm/ft, which is pretty much the same, and for all practical purposes, the gauge is correct for it.
Looking up "HO Gauge", or any of the "XX Gauges" in Wikipedia will redirect to "HO Scale".
