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OP: Why couldn’t traditional Chinese Yinpiao银票/silver drafts be forged if they were merely slips of paper? (cr大明宝钞，渐越)

Traditional Chinese yinpiao/silver drafts were paper vouchers issued by private banks starting from the Song Dynasty(960–1279). People could exchange these slips for physical silver at bank branches across the country.

Silver drafts were made in multiple copies with matching serrated seal edges. One copy went to the customer and others stayed at the bank. All edges had to fit perfectly together to withdraw silver. The unique split edge marks were almost impossible to copy.

This mechanism is known as qifeng骑缝 (split-joint seal) in China. It first originated in the Western Zhou Dynasty (1046–771 BC). The Rites of Zhou records that contracts were written on bamboo or wooden slips in duplicate. Notches and marks were carved in the middle before splitting the slips, with each party keeping one half. The two halves would be matched by their notches for verification.

During the Spring and Autumn and Warring States periods (770–221 BC), this idea evolved into hufu虎符/tiger tally tokens. A military tally was split into two pieces with identical inscriptions carved along the split edge. Troops could only be deployed if the patterns and characters on both halves perfectly aligned, serving as a metal version of the split-joint anti-counterfeiting system.

The technology matured in the Tang Dynasty (618–907). Government documents and private contracts commonly used split-joint seals stamped across the dividing line. The Chinese character "hetong合同" (contract) was written across the middle before the paper was torn apart, so the complete characters would only appear when the two halves were put together. This split-coupon system was later adopted for Song Dynasty (960–1279) jiaozi paper money and yinpiao/silver drafts of the Ming and Qing dynasties (1368–1912).

Official Song dynasty paper money (Jiaozi交子) was abolished in 1107. Private silver drafts issued by Qing-era piaohao票行 (ancient exchange banks) vanished completely in 1951, hit hard by modern banks and currency reforms. Nowadays silver drafts no longer circulate as currency. Their collectible value depends on their rarity and physical condition.

First they were called "chirographs", but later the word "indenture" (in its earliest meaning as just a legal document of any kind between two people) came to be used, originating from the practice of a contract being written twice on a single piece of parchment and then cut in half with serrated edges (as in dent, "teeth" -> indents -> indenture) in order for each party to take one half, so they could later piece them together and verify that there had been no forgery -- same as the Chinese silver drafts!

(Charter of the Clerecía de Ledesma, 1252, showing the serrated indents at the top -- presumably they are cutting rather than tearing because they're using parchment, which I expect is much harder to tear than wood-pulp paper like the Chinese were using)