In the 1930s, the U.S. appraisal industry opposed the "mixing" of the races, which it believed would cause "the decline of both the human race and of property values." Appraisers ensured segregation through their property rating system. They ranked properties, blocks, and even whole neighborhoods according to a descending scheme of A (green), B (blue), C (yellow), and D (red). A ratings went to properties located in "homogenous" areas -- ones that (in one appraiser's words) lacked even "a single foreigner or Negro." Properties located in neighborhoods containing Jewish residents were riskier; they were marked down to a B or C. If a neighborhood had black residents it was marked as D, or red, no matter what their social class or how small a percentage of the population they made up. These neighborhoods' properties were appraised as worthless or likely to decline in value. In short, D areas were "redlined," or marked as locations in which no loans should be made for either purchasing or upgrading properties.The FHA embraced these biases. It collected detailed maps of the present and likely future location of African Americans, and used them to determine which neighborhoods would be denied mortgage insurance. Since banks and savings and loan institutions often relied upon FHA rating maps when deciding where to grant their mortgages, the FHA's appraisal policies meant that blacks were excluded by definition from most mortgage loans.The FHA's Underwriting Manual also praised restrictive covenants as "the surest protection against undesirable encroachment" of "inharmonious racial groups." The FHA did not simply recommend the use of restrictive covenants but often insisted upon them as a condition for granting mortgage insurance....the FHA effectively standardized and nationalized the hostile but locally variable racial biases of the private housing industry.