A watermark is a design "embedded" in a sheet of paper and may be found on fine paper stock available at most stationers, on paper stock used to make currency and banknotes, and for our purposes, on certain postage stamps. A watermark can depict almost any design, but the U.S. chose to use only three letters, "U", "S", and "P", to make all the watermarks on its postage stamps, although it did accidentally use the letters "I" and "R" on three of its regularly issued stamps, see below.

Paper is made by allowing a thin layer of paper pulp to dry over a wire mesh. Early watermarks on hand-made paper were made by either raising the wire mesh in the area of the desired design or more often by attaching an additional thin wire mesh shaped in the design of the watermark. This effectively made the paper in the area of the watermark thinner than the surrounding paper and standout when held to a bright light since it transmits light more readily. Watermarks on hand-made paper usually stand out quite well when held to strong light.

With machine-made paper, from which most U.S. stamps are printed, the process is somewhat different. The watermark is added by applying a cylindrical device called a "dandy roll" to the paper after the paper has already begun to dry, resulting in watermarks that are not as pronounced and therefore more difficult to detect. Most watermarks on U.S. stamps will not be visible unless placed in a dark tray with watermark fluid, The watermark appears darker since it is thinner than the surrounding stamp and lets the reflection from the dark tray through more readily.

The watermark design was added many times in a regular pattern to make a single sheet of printable paper stock, the double-line in regular rows and columns and the single-line in an offset diagonal. The watermark pattern was independent of the size and design of the stamp and can therefore appear in just about any position on any given stamp, but must always be either parallel or perpendicular to the design of the stamp.

To get an idea of where the watermark can appear on a stamp, cut a small rectangular window the size of the stamp out of a small piece of cardboard and place this cutout over one of the printable watermark templates. From the cutout at right the bottom of the letter "S", the top right of the letter "P" and the top left of the letter "S" will be on the stamp. It is fairly easy to verify that on a normal-sized Washington-Franklin stamp, at most three letters are possible on a single-line watermarked stamp and four letters on the double-line watermarked stamps. 

Watermarked "Error" Stamps

There were three regularly issued U.S. postage stamp inadvertently printed on the Internal Revenue "USIR" watermarked paper. The 6c and 8c First Bureaus (Triangles), Scott's 271a and 272a, were accidentally printed on this stock. They must have either a clear "I" or a clear "R" to be certified genuine since the "U" and the "S" of the "USIR" watermark are for practical purposes indistinguishable from the "U" and the "S" of the double-line "USPS" watermark. On the other watermark error, the $1 Wilson Prexie, Scott 832b,  any watermark including the "U" and the "S", will confirm the error since the regularly issued stamp was was not printed on watermarked paper. 


Faked Watermarks

It is important to note that watermarks are sometimes faked to "create" the more expensive counterpart and sometimes to hide a thin. Knowing where the watermark can and cannot appear may help to catch this problem. The fake watermark is made by scraping away some of the stamp paper in the shape of a watermark, creating a thinned area on the stamp. Thins can be distinguished from genuine watermarks by the way the stamp dries after having been submersed in fluid. A thin will "whiten out", a genuine watermark will not. This scraping method will not work with genuinely gummed stamps, since the scraped area will be obvious. However, it is possible to remove the gum, scrape the stamp in the pattern of a watermark and then regum the stamp. Again the stamp will "whiten out" in the area of the thin.