Model railroads originally used the term gauge, which refers to the distance between the rails, just as full-size railroads continue to do. Although model railroads were also built to different gauges, standard gauge in full-size railroads is 4' 8.5". Therefore, a model railroad reduces that standard to scale. An HO scale model railroad runs on track that is 1/87 of 4' 8.5", or 0.649" from rail to rail. Today model railroads are more typically referred to using the term scale instead of "gauge" in most usages.