The agencies assess their the ability to pay off loans or investments and then rate them on a sliding scale, ranging from AAA to D.

CNN: Explainer: The power of credit rating agencies

The highest-flying markets in recent weeks have been for bonds issued by the least creditworthy companies: those rated B and lower. (For comparison, the highest is AAA and D stands for default.) The Merrill Lynch index of B-rated corporate bonds now yields 7.4% a quarter of a point less than a month ago, even though Treasury-bond yields are higher.

ECONOMIST: Buttonwood

According to the AAA in Washington, D.

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