The correct answer is: paper money
In the medieval times, the wealthy people deposited their gold with the goldsmiths. They kept it safe from the thieves and when money was required they gave it to the owner. As time passed by, they evolved, depeloping banking in its modern form. Initially they worked with receipts, the goldsmith counted the gold and gave the recept to the owner, charging some gold for this service, of course. Later on, people used those receipts to pay for things (instead of using the coins) as they represented an exact amount of gold. As you can see, this developed on the paper money, as it was easier and safier to carry and made transactions.
