If someone cosigns on a mortgage for a person with bad credit, whose credit score do they look at?

Question by Eric D: If someone cosigns on a mortgage for a person with bad credit, whose credit score do they look at?
Will the interest rate be based on the cosigner's good score, the borrower's bad score, or a blend of both?

From my limited research, I have seen conflicting viewpoints. One person says it will be based on the lower score. Another person says having a cosigner will improve the interest rate offered. What is the truth?

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