In its simplest form, a quarter-circle of metal, ivory or wood, with a pair of sights along its upper edge, a scale of degrees inside its curved edge, and a light plumb-bob on a string hanging from its right-angled corner. It is basically an instrument for measuring altitude, but different models have been developed for a variety of purposes. A quadrant intended solely or almost solely for finding the time is termed a 'horary quadrant'; it is in effect an altitude sundial. Horary quadrants have been made to indicate unequal or equal hours, and sometimes both. There are two methods of arranging for a date setting. In one, the date scale is marked vertically near one straight edge of the quadrant, and a slider on the plumb-bob line is set to this. Then a sight of the sun is taken and the string held against the quadrant face with the thumb, the slider giving the reading on the curved hour lines. In the other method, the date scale is near the curved edge of the quadrant, the line is set to the date and its slider moved to the point where it meets the 12 line, the same subsequent procedure being followed. A fine horary quadrant indicating unequal hours, dating from the early 14th century, is in Merton College, Oxford. The earliest surviving dated quadrants showing equal hours are one of 1398 in Dorchester Museum and a similar example of 1399 in the British Museum. A development of the simple horary quadrant was the so-called 'Gunter' quadrant, invented c. 1618 by Edmund Gunter of Christ Church, Oxford. This is engraved with two sets of lines side by side on the face of the quadrant. The set to the left forms a horary quadrant while that to the right enables the sun's azimuth to be found when its altitude is known at a given date. An unusual feature is that both sets of lines are folded over at the equinox line to permit a more open scale, but giving a complex appearance of crossed lines. Other special forms of quadrant are the sinical quadrant, whose scales enable the sines of the measured angles to be read off directly, and gunnery quadrants, from which the angle of elevation of the target and the necessary elevation of the gun barrel can be determined. Very large fixed quadrants for accurate measurement of the altitude of stars were essential pieces of equipment in pioneer astronomical observatories.