They respectively attach risk weights to their mortgage books of 15%, 15%, 14% and 11%.

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The temptation is to add further measures to restrict their wiggle-room (Basel 3 uses both risk weights and a leverage ratio).

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For risk-based capital purposes, the risk weights for Treasury securities and other securities issued or guaranteed by the U.S. government, government agencies, and government-sponsored entities will not change.

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On the one hand, the ratio of capital to assets would be boosted very significantly in "good" RBS, because some loans and investments with very high risk weights (in the jargon) would have been detached.

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First, under the Basel system that allows big banks with "advance" status to set their own evaluations of the riskiness of different categories of loan - the so-called risk weights - there is not enough consistency in how much capital different banks hold to cover similar risks.

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It is all down to our old friend, the Basel Rules, which attach risk-weights to different categories of loans.

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For measuring credit risk, banks will have three options, each of which will assign more detailed weights to their assets than Basel 1 did.

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