A watermark is a distinguishing mark or design incorporated in paper during the making. The mark may be incorporated by die impression or rolling, or by variation in the mesh used for making laid paper. The mark varies in thickness and is thus translucent and visible when the paper is illuminated from the verso.

Watermarks have been used in fine book papers for centuries, worldwide, and now can serve as bibliographic points to identify editions or states of a book. Bibliophiles and booksellers can access resources that catalog, codify, and identify watermarks by image, symbol, figure, text, coats of arms, etc. The following Web sites are in various languages.

Watermarks searchable by description:

https://memoryofpaper.eu/BernsteinPortal/appl_start.disp# 

http://www.wm-portal.net/niki/index.php 

Watermarks cataloged or described by figure, image, or text:

https://www.wasserzeichen-online.de/wzis/struktur.php 

International sources of watermark catalogs, databases, and library holdings:

http://www.paperhistory.org/Watermark-catalogues/ 

Watermark terminology:

https://rbms.info/vocabularies/paper/th35.htm 

Organizations for standardization of cataloging and archiving of watermarks:

http://abacus.bates.edu/wmarchive/wm-initiative/