This type of instrument is called a quadrant because it is shaped like a quarter-circle. It may be regarded as an astrolabe folded back twice on itself. This folding process yields the configuration of the planisphere on the front, and the overlay of the hour lines on the shadow square on the back of the quadrant. The limb, or graduated rim, is reduced to a single 90° arc, while the sights are mounted in a fixed position on one of the straight sides. To the right angle is attached a small plumb line for reading the angular altitude of the stars on the limb. As in the astrolabe, the shadow square was used for terrestrial measurements, while the hour lines enabled the instrument to be used as a sundial. Time reckoning could be confined to a single latitude as in the case of the two-limbed hour quadrant that showed the hours on a second limb and the signs of the zodiac on one of the straight sides. Alternatively, time reckoning could be extended to all latitudes, as in the universal quadrant with cursor, which enabled the user to shift the zodiac band along the graduated arc. Latitude could be easily determined by measuring the altitude of the Sun or North Star; that was the instrument's main application in navigation. The main variants of the quadrant were designed for the same purpose. They included the sextant, which measures the sixth part of a circle; the octant, which measures the eighth part; the sinical quadrant, also known as primum mobile quadrant, for trigonometric calculations; and the Davis quadrant, composed of two complanar graduated arcs that allowed the user to measure solar altitude without having to look at the Sun directly.