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collateralized mortgage obligations
I think the definition of overcollateralization on investopedia will answer this question for you.  Namely this part: For example, in the case of a mortgage backed security, the principal amount of an issue may be $100 million while the principal value of the mortgages underlying the issue may be equal to $120 million. The bond is packed with more mortgages than the face value indicates.  It's effectively sold at a discount to underlying value.