Banks

In medieval times the first banks appeared that were the mediators between the depositors and those who wanted to borrow money.

Bankers were taking the real coins of people with the certain interest and borrowed it to other people taking the higher interest getting profits from such operation. The depositor got the certificate that confirmed his deposit and was the proof that real money is in the bank and when the depositor presented this paper he/she received the real money.

With the lapse of time these papers got the same value as real money. It is necessary to mention that these papers were the requirements of real money from the bank and all of them had the real money securing on all 100% (at least such system existed in the beginning).

People who borrowed money in the bank wrote the note that they are obliged to return the debt cum interest. In exchange for this note they got either money or the certificates. The majority of people prefered to take the certificates to money. And the real money was only the "dead" money in the bank.

Later banker discovered that they are able to give the loans using the "dead" money that was in fact the unnecessary money and in such way could increase their profits. In such case the number of certificates increased the number of real money. And such secret of banks could be discovered only if all depositors asked the bankers to return them the money presented in certificates. In such case bank wouldn't simply have enough money to return it.