The artefacts in question were humble sticks of willow, about eight inches (20cm) long, called Exchequer tallies. The willow was harvested along the banks of the Thames, not far from the Palace of Westminster in central London. ... Tallies were a way of recording debts with a system that was sublimely simple and effective. The stick would contain a record of the debt, for example: "£9 4s 4d from Fulk Basset for the farm of Wycombe". Fulk Basset was a Bishop of London in the 13th Century. He owed his debt to King Henry III.
Now comes the elegant part. The stick would be split in half, down its length from one end to the other. The debtor would retain half, called the "foil". The creditor would retain the other half, called the "stock" - even today, British bankers use the word "stocks" to refer to debts of the British government. Because willow has a natural and distinctive grain, the two halves would match only each other.
Of course, the Treasury could simply have kept a record of these transactions in a ledger somewhere. But the tally stick system enabled something radical to occur. If you had a tally stock showing that Bishop Basset owed you £5, then unless you worried that he wasn't good for the money, the tally stock itself was worth close to £5 in its own right.
If you wanted to buy something, you might well find that the seller would be pleased to accept the tally stock as a safe and convenient form of payment. So the tally sticks themselves became a kind of money, a particular sort of debt that could be traded freely, circulating from person to person until it utterly separated from Bishop Basset and a farm in Wycombe.