Collateralized debt obligation

Mezzanine tranches started to lose value in 2007, by mid year AA tranches were worth only 70 coins. By October triple-A tranches had started to fall. Regional diversification notwithstanding, the mortgage backed securities turned out to be highly correlated

In the summer of 2006, the Case–Shiller index of house prices peaked. In California, home prices had more than doubled since 2000 and median house prices in Los Angeles had risen to ten times the median annual income. To entice those with low and moderate income to sign up for mortgages, down payments and income documentation were often dispensed with and interest and principal payments were often deferred upon request.

To deal with the problem, investment bankers "recycled" the mezzanine tranches, selling them to underwriters making more structured securities—CDOs. Though the pool that made up the CDO collateral might be overwhelmingly mezzanine tranches, most of the tranches (70 to 80%) of the CDO were rated not BBB, A-, etc., but triple A. The minority of the tranches that were mezzanine were often bought up by other CDOs, concentrating the lower rated tranches still further.

Subprime mortgages had been financed by mortgage-backed securities (MBS). Like CDOs, MBSs were structured into tranches, but issuers of the securities had difficulty selling the more lower level/lower-rated "mezzanine" tranches—the tranches rated somewhere from AA to BB.

The rise of "ratings arbitrage"—i.e., pooling low-rated tranches to make CDOs—helped push sales of CDOs to about $500 billion in 2006, with a global CDO market of over US$1.5 trillion. CDO was the fastest-growing sector of the structured finance market between 2003 and 2006; the number of CDO tranches issued in 2006 (9,278) was almost twice the number of tranches issued in 2005 (4,706).

In 2005, as the CDO market continued to grow, subprime mortgages began to replace the diversified consumer loans as collateral. By 2004, mortgage-backed securities accounted for more than half of the collateral in CDOs. According to the Financial Crisis Inquiry Report, "the CDO became the engine that powered the mortgage supply chain", promoting an increase in demand for mortgage-backed securities without which lenders would have "had less reason to push so hard to make" non-prime loans. CDOs not only bought crucial tranches of subprime mortgage-backed securities, they provided cash for the initial funding of the securities. Between 2003 and 2007, Wall Street issued almost $700 billion in CDOs that included mortgage-backed securities as collateral.