A horizontally held bar carrying two easily visible marks a fixed, known distance apart, and used for determining the distance from an observer to the bar by means of the angle subtended at the observer by the marks on the bar. The marks are often placed on fittings mounted at the ends of the bar and connected by invar wires under slight but constant tension. This counteracts the effect of temperature on the distance between marks. In use, the subtense bar is mounted horizontally on a tripod and centred over the mark to which distance is to be determined. The angle subtended at the observing instrument is measured and automatically converted by the instrument to distance. The observer reads off the distance, not the subtended angle.