First introduced in 1862 and remaining in circulation until 1971, United States Notes, or Legal Tender Notes as they are also called, were the original "greenbacks" thanks to their green, reverse sides. At the time of their debut, the notes were controversial because they were not backed by gold. In other words, the paper was valuable simply because the government said it was.

The earliest United States Notes look a bit like stock certificates, with their detailed, patterned engraving and formal appearance. Collectors seek out these earliest notes, of course, but they also go for the so-called Rainbow Notes of 1869, which were colored in green, red, and blue, and had red and blue jute fibers in the paper to prevent counterfeiting.

The currency was redesigned several times in the 1870s and in 1880. A much beloved note is the $10 bill from 1901, whose front features a buffalo flanked by portraits of Lewis and Clark. A few years later, in 1907, the same image of a pioneer family that had graced the fronts of 1869 United States Notes also appeared on that year’s $5 bill...