"The key idea behind a CDO is that there will always be at least some money in the box, regardless of how dicey the individual assets inside it are. No matter how you look at a single unemployed ex-con trying to pay the note on a six-bedroom house, he looks like a bad investment. But dump his loan in a box with a smorgasbord of auto loans, credit-card debt, corporate bonds and other crap, and you can be reasonably sure that somebody is going to pay up. Say $100 is supposed to come into the box every month. Even in an apocalypse, when $90 in payments might default, you'll still get $10. What the inventors of the CDO did is divide up the box into groups of investors and put that $10 into its own level, or "tranche." They then convinced ratings agencies like Moody's and S&P to give that top tranche the highest AAA rating - meaning it has close to zero credit risk."