If mortgage rates decline significantly, borrowers may decide to prepay the principal on their loan even if they face prepayment penalties. One way that lenders protect themselves from prepayments in such circumstances is by requiring the borrower who prepays to purchase for the lender a set of U.S. Treasury securities whose coupon payments replicate the cash flows the lender will lose as a result of the early retirement of the mortgage. This process is referred to as: Yield Maintenance Lock out Defeasance O Curtailmaent