During the incunabula period, Europeans used rags to make paper by the following method: the rags were cut into small pieces; fermented; ground by watermill; and scooped into a mould to dry. Therefore handmade paper does not have a uniform thickness; it varies in thickness according to the mesh of the mould. If held against the light, the thinner part of handmade paper appears brighter, and it is possible to detect thick lines spaced several centimeters apart, as well as thin lines closely spaced crossing the thick lines at a right angle. The thick lines are referred to as the "chain line" and the thin lines the "wire line."

Chain lines are visible lengthways in a whole sheet of paper. If the paper is folded, the direction of chain lines and wire lines varies as shown in the following table: