from Bookbinding and the Conservation of Books A Dictionary of Descriptive Terminology
A distinguishing letter, design, symbol, etc., incorporated into a paper during manufacture. True watermarks are a localized modification of the formation and opacity of the paper while it is still wet, so that the marks can be seen in the finished sheet of paper when viewed by transmitted light.
chain lines ( chain marks ) n.
The widely spaced watermark lines, about 25 mm apart, parallel to the shorter sides of a sheet of laid paper, caused by the “chain wires,” i.e., the wires to which the finer laid wires of the MOLD (1) are attached for support.