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Italian gauge refers to three different sizes of track gauge, all used exclusively in Italy:
- 1,445 mm (4 ft 8 7⁄8 in), called Italian broad gauge
- 950 mm (3 ft 1 3⁄8 in), called Italian metre gauge
- 700 mm (2 ft 3 9⁄16 in), called Italian narrow gauge
These gauges were legally defined. Italian law defined its track gauges from the centres of each rail, rather than the inside edges of the rails, giving some unusual measurements. According to the law of 28 July 1879, the only legal gauges in Italy were 1,500 mm (4 ft 11.06 in), 1,000 mm (3 ft 3.37 in) and 750 mm (2 ft 5.53 in) measured to the middle of the rail, which corresponds to 1,445 mm (4 ft 8 7⁄8 in), 950 mm (3 ft 1 3⁄8 in), and 700 mm (2 ft 3 9⁄16 in) inside the rail.
The middle size has 1,000 mm between the centres of the rails, which explains why it is called Italian metre gauge, in spite of it in fact being 950 mm in gauge when measured from the inside of the rails in the way gauges usually are measured in other countries.
A disadvantage of measuring from the centre of the rail is that the width of the rail varies, affecting the gauge. It is easier and more reliable to measure from the inner edges of the rails.