As its name suggests, the quadrant consists of a quarter-disk whose arc is divided into 90 degrees. One right side carries two perforated sights. A plum bob hangs from the apex where the right sides meet. The position of the string on the rim (called the limb) indicates the inclination of the line of sight relative to the horizon. If the observer aims at the North Star in the tail of the Small Dipper (Ursa Minor) during the navigation, the altitude measured gives a rough indication of the latitude reached. If, instead, the quadrant is aimed at the Sun, the latitude of the location is calculated by applying an algebraic formula to the Sun's maximum altitude above the horizon. The latitude is equal to the 90-degree angle minus the altitude measured, plus the declination reached by the Sun on the day of the observation. The latter value was given for every day of the year by astronomical tables.