The study looks at how rules about how much cash banks must keep on hand affect the banking system. They found that the current rule has made banks safer but also reduced their ability to lend money. Banks not following the rule may face more risks. A new model suggests that the rule can help but also cause problems because the cash buffer acts like a tax and is costly to maintain. The rule can make the cost of cash fluctuate more. A new system with a central bank could improve the current rule by using prices instead of fixed amounts.