Omega_Einhorne saidwhats watermark?
lol I know right.
No idea what watermarking is.
It was originally faint shadow marking applied to paper during its manufacturing, that was best seen when light passed through the paper. It was mostly used to identify the paper manufacturer, but could be customized for business, personal and government use. Once fairly common on better business paper, and on expensive parchment stationary mostly for ornament.
Today's computers can print a simulation of that look, and it's still called watermarking, even though it's on the surface, and not a modification of the paper pulp itself, as in the original wet process. As the thread explains, now it's applied to photographs and other documents to identify copyrighted material that cannot be legally copied. And because it's done across the photo itself, rather than along a border, it's difficult for an unauthorized user to crop out.
Here's a watermarked US Government envelope from the 1880s. The color makes this easier to see than on white paper. I'm not sure if the public could buy these, or only for official use. The ornate intertwined letters are: USPOD. (United States Post Office Department), which perhaps means that's where you bought them.