I've always divided US philately into seven subsections, largely for ease of study and because the characteristics of the sta
mps from section to section are so different. The divisions are stamps to 1869, the Banknotes(1870- 1893), Bureau Issues period (1893-1912), Washington Franklins, modern, Back of the book and Revenues. Each of these sections has various predominant elements that distinguishes it from the others and though serious philatelists may argue as to which are the most predominant characteristics of any of the subsections, I think most would agree with me that there is enough differentiation between the various sections that they can be discussed separately. Banknotes though have long been the wall flower of US philately.
The Banknote issues came out in 1870 and were the regular postage stamp issues of the United States until 1893. They are called the Banknotes because they were printed by private contractors, successively The National, Continental and American Bank Note Companies. The stamps were in use for nearly 25 years during perhaps the greatest commercial expansion in American history. Scott divides them into 94 major catalog numbers (#135-229) and they have consistently been the least popular of all the m
ajor divisions of US philately. The reasons for this are the relative unattractiveness and similarity of the stamps. The colors fade (and were drab to begin with), and the fact that the four major issues of Banknotes (#135-144, 145-155, 156-166, & 182-191) were all produced with the same plates with either very slight design changes (called secret marks) or grills or on different paper hasn't helped their popularity. Many of the stamps take more skill to identify than many collectors have, and thus these are often the most ignored of US issues.
But ignorance in philately can be bliss (at least the ignorance of other collectors). To a philatelist willing to invest some time and effort, Banknotes make a wonderful specialty where there are still some finds to be made.