: Historically, many paper currencies were backed by gold reserves until the mid-20th century; the US officially ended gold convertibility in 1971.
: Designs visible only when the note is tilted at specific angles.
: Promissory notes originated in 7th-century China during the Tang Dynasty to avoid carrying heavy metal coins.
: Modern systems use fuzzy logic, image processing, and machine learning to distinguish genuine notes from increasingly sophisticated fakes. IV. Economic Costs and Benefits Costs and Benefits to Phasing Out Paper Currency - NBER