Central bank balance sheets have become a monetary policy tool as central banks purchase bonds to keep long-term borrowing costs low. The chart on the left shows the evolution of the U.S. Federal Reserve's balance sheet after four rounds of quantitative easing across the different type of assets they buy. The chart on the right looks at balance sheets in terms of their size relative to their economy, as even though absolute size of the purchases may be smaller, their impact can still be meaningful.