All hand-made paper was made on paper moulds, consisting of a rectangular wooden shape and a metal grid. The wet material (usually cotton rags) would have been pressed into this mould, which would leave its marks in the paper in the form of watermarks. When you hold such a piece of paper up to the light, you can see the chain-lines, running horizontally (parallel to the short edges of the paper), crossed by wire-lines, running vertically (parallel to the long edges of the paper).
Different paper formats have different characteristics of chain-line direction. This is due by the different ways in which the paper would be folded after it was produced. A sheet of paper folded in folio, for example, has the chain marks running vertically instead of horizontally.
Occasionally turned chain-lines are found. These are chain-lines that appear to run in the wrong direction for a particular format. The mostly occur in books produced in the late seventeenth and in the eighteenth century. Turned chain-lines have two different causes. Either double-sized sheets of paper would be cut in half before printing, or sheets would have been made in side-by-side two-sheet moulds.
Contributed by Isha van der Burg.