Paper manufacturers design incorporated during papermaking. In the process, a wire design, e.g. with the initials or crest of the manufacturer, is laid on the wire mould on top of which the pulp is spread. On the spot where the design lay under the pulp, the finished paper is thinner. When held up to the light, the mark is visible through the paper. In Beethovens times, a specific wire design could only be used for a very short period. Thus the watermark is helpful for dating paper.