In the early 1980s, Freddie Mac introduced collateralized mortgage obligations (CMOs), which separated the payments from a pooled set of mortgages into "strips" carrying different effective maturities and credit risks. Since 1980, the outstanding volume of GSE mortgage-backed securities has risen from less than $ billion to more than $ trillion today.
Alongside these developments came the establishment of private mortgage insurers, which competed with the FHA, and private mortgage pools, which bundled loans not handled by the GSEs, including loans that did not meet GSE eligibility criteria – so-called nonconforming loans. Today, these private pools account for around $ trillion in residential mortgage debt.