It was a simple most ancient clock for which very early testimonies of its use by the Egyptians exist.
A horizontal gnomon placed perpendicularly above a horizontal bar with 5 calibration marks indicated the 5 ante meridian day hours (1st, 2nd, 3rd, 4th and 5th) if the device was turned to the West and the 5 post meridian day hours (7th, 8th, 9th, 10th and 11th) if it was turned to the East. The indication of midday (6th) coincided with the base of the gnomon. It was not possible for the sunrise and the sunset to be indicated with this device.