an alt-a mortgage, short for alternative a-paper, is a type of u.s. mortgage that, for various reasons, is considered riskier than a-paper, or "prime", and less risky than "subprime," the riskiest category. for these reasons, as well as in some cases their size, alt-a loans are not eligible for purchase by fannie mae or freddie mac. alt-a interest rates, which are determined by credit risk, therefore tend to be between those of prime and subprime home loans, although there is no single accepted definition of alt-a. typically alt-a mortgages are characterized by borrowers with less than full documentation, average credit scores, higher loan-to-values, and more investment properties and secondary homes. a-minus is related to alt-a, with some lenders categorizing them the same, but a-minus is traditionally defined as mortgage borrowers with a fico score of below 680 while alt-a is traditionally defined as loans lacking full documentation. alt-a mortgages may have excellent credit but may not meet underwriting criteria for other reasons. during the past decade, a significant amount of alt-a mortgages resulted from refinancings, rather than property purchases.