For example, in September 2008, when a prominent money market fund “broke the buck” (that is, its net asset value fell below one dollar), the Treasury Department initiated a temporary guarantee program to avert a run on other money market mutual funds.
After that, commercial banks came into existence, the planned economy was replaced by market economy, and the interest rate determined by market supply and demand came among. At this point, an important question is, “What is the appropriate level of the interest rate?” We can see that central banks in various countries all concentrate on the adjustment and control of the interest rate.