AIG is saying here that it has insured $307bn of corporate loans and prime residential mortgages that are on the balance sheets of banks, mostly European banks.
The banks have bought this insurance to protect themselves against the risk that these loans would go bad, that borrowers would default.
Their motive for doing so was to reassure their respective regulators - such as the FSA for UK banks - that these loans are of minimal risk.
And the benefit of doing that was that they could lend considerably more relative to their capital resources.
But if AIG is in trouble, then doubts arise about whether it would be able to honour the financial commitments it has made through these insurance contracts (which, for those of you who like to learn the lingo, are called super senior credit default swaps).
In fact, in a wholly mechanistic way, the downgrades of AIG's credit rating that we saw last night automatically increased the perceived riskiness of loans made by banks that have insured credit with AIG.
Which means those banks' balance sheets become weaker - and that could mean that they'll be forced by their regulators to raise additional capital.
So there's a widespread view among bankers that the US Treasury and the Federal Reserve simply can't allow AIG to fail, in the way that they felt that they could allow Lehman to collapse into insolvency.
If AIG went down, a number of banks' balance sheets would be mullered - there would a dangerous risk to the stability of the global financial system.
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