A collateralized debt obligation (CDO) is a type of structured asset-backed security (ABS). Originally developed as instruments for the corporate debt markets, after 2002 CDOs became vehicles for refinancing mortgage-backed securities (MBS). Like other private label securities backed by assets, a CDO can be thought of as a promise to pay investors in a prescribed sequence, based on the cash flow the CDO collects from the pool of bonds or other assets it owns. Distinctively, CDO credit risk is typically assessed based on a PD derived from ratings on those bonds or assets. The CDO is "sliced" into "tranches", which "catch" the cash flow of interest and principal payments in sequence based on seniority. If some loans default and the cash collected by the CDO is insufficient to pay all of its investors, those in the lowest, most "junior" tranches suffer losses first. The last to lose payment from default are the safest, most senior tranches. Consequently, coupon payments (and interest rates) vary by tranche with the safest/most senior tranches receiving the lowest rates and the lowest tranches receiving the highest rates to compensate for higher default risk. As an example, a CDO might issue the following tranches in order of safeness: Senior AAA (sometimes known as "super senior"); Junior AAA; AA; A; BBB; Residual.Separate special purpose entities—rather than the parent investment bank—issue the CDOs and pay interest to investors. As CDOs developed, some sponsors repackaged tranches into yet another iteration, known as "CDO-Squared", "CDOs of CDOs" or "synthetic CDOs".In the early 2000s, the debt underpinning CDOs was generally diversified, but by 2006–2007—when the CDO market grew to hundreds of billions of dollars—this had changed. CDO collateral became dominated by high risk (BBB or A) tranches recycled from other asset-backed securities, whose assets were usually subprime mortgages. These CDOs have been called "the engine that powered the mortgage supply chain" for subprime mortgages, and are credited with giving lenders greater incentive to make subprime loans, leading to the 2007-2009 subprime mortgage crisis.