From Wiki:
The 45 mm gauge originated from '1 gauge' or 'Gauge One' which was first used in Europe and England and used to model standard gauge trains in the scale of 1:32.
LGB were first to adopt the term 'G Scale' and used the gauge of 45 mm (1.772 in) to model 1,000mm gauge European trains in 1:22.5 scale.
Below are some typical scales with more specific terms that all run on 45 mm gauge track:
Gauge One/ 1 Gauge; scaled at 1:32 (3/8" to the foot) used to model standard gauge trains of 4 ft 8 1⁄2 in (1,435 mm) standard gauge gauge.
'A' scale; Uncommon term for 1:29 scale ]first used by AristCraft for modeling standard gauge prototypes on 45mm track. Incorrect scale/gauge but proportionally a similar size to other popular brands of the time.
'G' scale; 1:22.5 used to model European trains that run on 1,000 mm (3 ft 3 3⁄8 in) metre gauge track. 'G' term is thought to be originally from 'Gross' or 'Garden' by LGB who popularized the scale.
'H' scale (Half inch) 1/2" to the foot scale; 1:24 used to model 3 ft 6 in (1,067 mm) gauge or 'Cape gauge.' Incorrect scale used for 3 ft (914 mm) gauge track.
'F' scale (Fifteen) 15mm to the foot scale; 1:20.32 Correct scale/gauge typically used to model North American narrow gauge trains on 3 ft (914 mm) gauge track.
'Seven Eighths' 7/8" to the foot scale; 1:13.7 used to model trains on 2 ft (610 mm) narrow gauge track.
'16mm scale' 16mm to the foot scale; 1:19.05 Originally intended for modelling 2 ft gauge prototype railways on 32mm track (SM32). The models are often re-gaugable to also run on 45mm track. This scale has also been used to model 2 ft 6 in (762 mm) gauge prototype trains.
