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A watermark is a design manufactured into paper. For
stamps, the watermark is typically a small simple pattern that is
a slight thinning of the paper, so that that it appears lighter when
the stamp is held up to the light, or darker when viewed against a
Historically, only about 200 different watermarks have ever been used on stamps. They range from simple geometrical shapes, to familiar symbols, to letters and numbers. Typically any given watermark is used by only one issuer, although for instance the British Empire used the same watermarks across all of its colonies and other territories.
In StampData, each watermark has a dedicated page, on which you specify the commonly-used name of the watermark, and can provide a more detailed description. The name should be unique among watermarks (currently the software does not enforce this). You can also identify some images as images of watermarks, using the same identification process as for stamp; the watermark's page then displays those images.
Every type of stamp must have a watermark specified, or "unwmk" to indicate that the paper does not have any watermark.