Credit enhancement is key in creating a security that has a higher rating than the underlying asset pool. Credit enhancement can be created, for example, by issuing subordinate bonds. The subordinate bonds are allocated any losses from the collateral before losses are allocated to the senior bonds, thus giving senior bonds a credit enhancement. As a result, it is possible for defaults to occur in repayment of the underlying assets without affecting payments to holders of the senior bonds. Also, many deals, typically those involving riskier collateral, such as subprime and Alt-A mortgages, use over-collateralization as well as subordination. In over-collateralization, the balance of the underlying assets (e.g., loans) is greater than the balance of the bonds, thus creating excess interest in the deal which acts as a "cushion" against reduction in value of the underlying assets. Excess interest can be used to offset collateral losses before losses are allocated to bondholders, thus providing another credit enhancement. A further credit enhancement involves the use of derivatives such as swap transactions, which effectively provide insurance, for a set fee, against a decrease in value.