 When the frame was pulled from the vat, these ribs produced a slight suction which pulled the water out of the newly formed sheet, and a slightly thicker layer of pulp was left across the top of the ribs.In a dried sheet, darker strips can be seen running along these chain lines when the sheet is held to the light.Improvements in mould making in the early 1800s lifted the chain wires slightly, resulting in a more evenly toned sheet.