Attended a coin show this past weekend to kill some time. Several take-aways from it, but this one was interesting and related. Talked with a dealer who had a lot of US currency for sale. You're probably familiar with the colored seals on US banknotes. I saw a new - to me - color there: Brown. I forgot what he called them (treasury notes???), but basically if you were a bank and deposited collateral with the treasury department, they'd print and issue the bank currency notes with your bank's name on them that you could issue and circulate. They were around in the early part of the 20th century. After that came yellow seals (gold certificates), blue seals (silver certificates), red seals (US notes) and green seals (federal reserve notes). Guy offered commentary that they were - respectively - backed by: Collateral; Gold; Silver; Debt; and, Thin Air. I thought it an apt and succinct description.