OO, HO and N are the three most popular model railway gauges worldwide. Z would seem to be a relatively obscure one, at least in the UK; it's one of the gauges specified by the US National Model Railroad Association (NMRA), and there is also a Z gauge used in Germany.
OO is the most popular gauge in the UK, but HO is more popular elsewhere. Hornby, the UK's principal manufacturer, uses OO gauge. OO and HO models use the same gauge track, so they can run on each other's layouts.
OO, at 1:76, is the largest gauge; HO is slightly smaller at 1:87. N gauge models are around half the size of either of these, at 1:148 in the UK and 1:160 elsewhere.
Wikipedia lists over 70 different model railway scales (or gauges), including ten that are UK–specific.
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