Note: Gauge means the distance between the rails. e.g. 32mm.
Scale means the ratio between the full size item and the model. e.g. 16mm. to the foot scale or 1/19th scale.
The scale and gauge known as "G" was introduced in 1968 by L.G.B. ( Lehmann Grosse Bahn - Lehmann's Big Train ) The scale is 13.5mm. to the foot or 1/22nd scale. (actually 1/22.5)
It has a track gauge of 45mm. which gives a full size equivalent of 1 metre gauge (3 foot 3 ins.) common to continental railways and known as "Narrow Gauge". It is roughly compatible with 1/20th - 1/24th scales.
Gauge 1 (such as Marklin) which is often advertised in the "G scale" section of eBay, is something entirely different.
It is also known as "10 Mil scale" and usually used to model standard gauge railways (4 foot 81/2 ins).
It has a scale of 9.5mm - 10mm to the foot or roughly 1/32nd scale. (equal to 'Britain's tractors' etc.)
It also runs on 45mm gauge track, but the smaller scale gives a full size equivalent track gauge of between 4 foot 6 ins. and 4 foot 9 ins. depending on how accurate your scale.
Modellers working in 16mm to the foot scale (1/19th) roughly compatible with 1/20th scale, have the choice of track gauges to work with. Either 32mm. known also as SM32 which gives a full size equivalent track gauge of 2 foot narrow gauge, common to Welsh industrial lines. Or 45mm (G gauge) giving a full size equivalent track gauge of just over 2 foot 9 ins.
Buyers should beware that most rolling stock advertised as "SM32, 16mm" is a lot narrower than the usual LGB or indeed Accucraft which can be compatible with both. You can usually tell from the photographs.
I hope this has been useful to modellers and eBayers new and old.
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