The euro-zone bailout of Greece is, in good part, a bailout of European banks. In France and Germany alone, banks hold some $90 billion worth of public and private Greek debt. The European Central Bank also holds Greek government debt, and the fear is that if Greece defaults, cascading losses could threaten all of Europe.

Are American banks also vulnerable? No one is sure. They are not big lenders to Greece, but they are big players in the derivatives markets. If Greece defaulted, a European bank holding a credit-default swap on Greek debt from an American bank would be entitled to a payout from that bank.

Credit-default swaps are the kind of derivatives that were behind the blowup of the American International Group and the near meltdown that followed in the global financial system. From the available evidence, it doesn’t appear that a Greek default would have the same destructive power, but no one is eager to test the proposition.

In his recent confirmation hearing to be the next leader of the European Central Bank, Mario Draghi, the central banker of Italy, warned that no one really knows who is on the hook for these risky financial instruments. “Who are the owners of credit-default swaps? Who has insured others against a default of the country?” he asked.