Volunteers measure and record each find. While she records, he, takes a plumb line to establish true vertical and measures the exact distance from the nearest side wall and the exact depth, in relation to fixed altitude measurements by the surveyor. The co-ordinates of the find locate it precisely within the square. The strings outline the walls, or balks, of the square excavation area. The unexcavated balks provide a walking area and a permanent vertical reference as the layers are dug out from within the square.