The most significant novel financial instruments relevant to this discussion are called Collateralized Debt Obligations, or CDOs. This type of security organizes the mortgages into several tranches that constitute a capital structure with some senior and Figure 2 CDO Issuance 1997-2007 Q1 some subordinated. The higher tranches' holders would get paid first in the event of default. Note that for a bank, securitizing a group of mortgages increases liquidity: the bank has cash in hand from selling the securities to invest in more real estate. CDO issuance absolutely SKYROCKETED with the housing bubble (see graph left), reaching a peak in 2006. Even more importantly, many of these newly issued CDOs were in fact CDO2s, or CDO3s. That's right, a CDO backed by CDOs backed by CDOs backed by subprime mortgages. Many traders and institutions could not be asked to check down through the many layers of financial abstraction to see that the mortgages beneath were bad assets. With the boom in CDO issuance, many portfolios were thoroughly exposed to the subprime crisis without awareness of the problem.