 From the 12th century on, the chain wires of a paper mould were attached directly to wooden ribs in the frame itself.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.