OO, HO and N are the three most popular model railway gauges worldwide.
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.
© Haydn Thompson 2017