John Doe must pay one of his creditors one hundred dollars and it would simplify matters very much if he could pay him with Richard Roe's promissory note. But the creditor is unwilling to accept the note in payment. Therefore, John Doe goes to his bank and sells Richard Roe's note to it after he has indorsed it by writing his name on the back of it. The bank gives John Doe one hundred dollars in gold or silver if he desires it, but he probably wishes to receive the money in the form of paper. Any one of several kinds of paper may be used to make the payment but it happens that it is made in bank notes. The bank note is a promissory note issued by the bank and essentially it is a promise to pay a certain sum of money on demand. John Doe's creditor will accept one hundred dollars in promissory notes of the bank because he knows that others will gladly accept the notes in turn from him.