Some say banks have been demanding German bonds as collateral for financing and refusing to accept the bonds of other euro-zone countries.

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If the ECB were to stop accepting Greek bonds as collateral for its lending to banks on the grounds that the bonds were in default, then Greece's banks, which were stuffed full of their government's bonds, would quickly run out of cash and collapse.

WSJ: European Disunion: Dithering at the Top Turned EU Crisis to Global Threat

NTT, Japan Tobacco and other companies as collateral for the bonds.

ECONOMIST: Rescuing Japanese banks