Unlike the original model formed by a cross-rod that can slide along a longer rod, these staffs present a cross-rod which is fixed to the end of the longer rod. The latter contains a third, thinner rod, which can be used as an extension. The two orthogonal rods represent, respectively, the base and height of a triangle of which the visual rays of the observer form the remaining sides. By means of these elements the instrument allows all celestial and terrestrial distances to be measured.