On September 20th the Fed will probably announce that it is putting QE into reverse. It does not intend to sell its assets. Rather, as its securities mature, it will stop reinvesting all of the proceeds. The permitted monthly “run-off” will gradually rise until it reaches $30bn for Treasury bonds, and $20bn for MBSs and housing-agency debt . The process will not be entirely predictable. Treasuries mature on a known date. But how fast the MBS portfolio shrinks will depend on how many Americans move house or refinance their mortgages (which in turn largely depends on interest rates).