A Roman surveying instrument which traced right angles. It was made of a horizontal wooden cross pivoted at the middle and supported from above. From the end of each of the four arms hung a plumb bob. By sighting along each pair of plumb bob cords in turn, the right angle could be established. The device could be adjusted to a precise right angle by observing the same angle after turning the device approximately 90 degrees. By shifting one of the cords to take up half the error, a perfect right angle would result. It was used for laying out the grid patterns of towns and forts, for road construction, and for centuriation.