The dastardly deeds of the credit default swap.

Who sells these CDSs? Banks, hedge funds, and AIG.

It's easy to see the attraction. Historically, bond issuers almost never go bankrupt. So, many banks and hedge funds figured they could make a fortune by selling CDSs, keeping the premium, and almost never having to pay out anything.

In fact, beginning in the late '90s, CDSs became a great way to make a lot more money than was possible through traditional investment methods. Let's say you think GE is rock solid, that it will never default on a bond, since it hasn't in recent memory. You could buy a GE bond and make, say, a meager 6 percent interest. Or you could just sell GE credit default swaps. You get money from other banks, and all you have to give is the promise to pay if something bad happens. That's zero money down and a profit limited only by how many you can sell.

Over the past few years, CDSs helped transform bond trading into a highly leveraged, high-velocity business. Banks and hedge funds found that it was much easier and quicker to just buy and sell CDS contracts rather than buy and sell actual bonds. As of the end of 2007, they had grown to roughly $60 trillion in global business.

So, what went wrong? Many CDSs were sold as insurance to cover those exotic financial instruments that created and spread the subprime housing crisis, details of which are covered here. As those mortgage-backed securities and collateralized debt obligations became nearly worthless, suddenly that seemingly low-risk event-an actual bond default-was happening daily. The banks and hedge funds selling CDSs were no longer taking in free cash; they were having to pay out big money.

Most banks, though, were not all that bad off, because they were simultaneously on both sides of the CDS trade. Most banks and hedge funds would buy CDS protection on the one hand and then sell CDS protection to someone else at the same time. When a bond defaulted, the banks might have to pay some money out, but they'd also be getting money back in. They netted out.