Rail gauge is the distance between the inner sides of the two parallel rails that make up a railway track. Fifty-five percent of the world's railways use a gauge of 4 ft 8½ in (1,435 mm), which is known as the standard or international gauge. Gauges wider than standard gauge are called broad gauge, those smaller are called narrow gauge. Some stretches of track are dual gauge, with three (or sometimes four) parallel rails in place of the usual two, to allow trains of two different gauges to share the same path. The term break-of-gauge refers to the situation at a place where different gauges meet.