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.