The primary source of credit enhancement is subordination, which creates a
hierarchy of loss absorption among the tranche securities. For example, if a trust
issued securities in 10 different tranches, the first (or senior) tranche would have
nine subordinate tranches, the next highest tranche would have eight
subordinate tranches and so on down the capital structure. Any loss of interest
and principal experienced by the trust from delinquencies and defaults in loans in
the pool are allocated first to the lowest tranche until it loses all of its principal
amount and then to the next lowest tranche and so on up the capital structure.
Consequently, the senior tranche would not incur any loss until all the lower
tranches have absorbed losses from the underlying loans.
A second form of credit enhancement is over-collateralization, which is the
amount that the principal balance of the mortgage pool exceeds the principal
balance of the tranche securities issued by the trust. This excess principal
creates an additional ―equity‖ tranche below the lowest tranche security to absorb
losses. In the example above, the equity tranche would sit below the tenth
tranche security and protect it from the first losses experienced as a result of
defaulting loans.
A third form of credit enhancement is excess spread, which is the amount that
the trust‘s monthly interest income exceeds its monthly liabilities. Excess spread
is comprised of the amount by which the total interest received on the underlying
loans exceeds the total interest payments due to investors in the tranche
securities. This excess spread can be used to build up loss reserves or pay off
delinquent interest payments due to a tranche security.