Watermark - a pattern embedded in paper during the production process, created by an intentional thinning of the paper.  Watermarked paper has been used for stamps, stamped envelopes, and postal cards.

The design of the watermark on stamps generally consists of the letters "USPS". There are two main types of "USPS" watermarks  —  Double-Line USPS and Single-Line USPS.

Double-Line USPS - a watermark in which the initials USPS are composed with double lines. In other words, the initials USPS are outlined and hollow.
Single-Line USPS - a watermark in which the initials USPS are composed with a single line.

The first watermark that appeared on U.S. postage stamps was the "double-line USPS", for "United States Postage Stamp". Period of use 1895-1910.

The "single-line USPS" watermark, introduced in 1910, was one of many experiments the Bureau of Engraving and Printing made to reduce waste due to paper shrinkage. Period of use 1910-1916.