The dastardly deeds of the credit default swap.

Banks all over the world bought CDS protection from AIG. If AIG is not able to make good on that promise of payment, then every one of those banks has lost that protection. Overnight, the banks have to buy replacement coverage at much higher rates, because the risks now are much worse than they were when AIG sold most of these CDS contracts.

In short, banks all over the world are instantly worth less money. The numbers seem to be quite huge-possibly in the hundreds of billions. To cover that instantaneous loss, banks will lend out less money. That means other banks can't borrow to pay this new cost, and weaker banks might not have enough; they'll collapse. That will further shrink the global pool of money.

This will likely spur a whole new round of CDS payouts-all those collapsed banks issue bonds that someone, somewhere sold CDS protection for. That new round of CDS payouts could cause another round of bank failures.

Generally, with enough time, financial markets can adjust to just about anything. This, though, would be an instantaneous transformation of the global financial system. Surely, the worst part will be the confusion. CDS are largely over-the-counter instruments. That means they're not traded on an exchange. One bank just agrees with another bank to do a CDS deal. There's no reliable central repository of information. There's no way to know how exposed a bank is. Banks would have no way of knowing how badly other banks have been affected. Without any clarity, banks will likely simply stop lending to each other.

Since we're only just now getting a handle on how widespread and intertwined they have become, it seems possible that AIG, alone, could bring the global economy to something of a standstill. It's also possible that it wouldn't.

John C. Hull, who literally wrote the book on credit derivatives, helped guide this Explainer