This instrument consists of a diameter cut away and two vertical sight vanes, with a horizontal one between. The diameter has a crossbar at centre which holds a raised section to accommodate the compass spigot. the back sight vane is a wide slit; it has two spring clips attached to it. The fore sight is an open arch with a thread, which travels along the diameter. The horizontal vane has both wide and narrow slits. There are also raised and inverted V sghts on top of the vertical vanes.