Home Owners’ Loan Corporation Redlining of Philadelphia
WikiPedia on Redlining:

Redlining is the practice of denying or increasing the cost of services, such as banking, insurance, access to jobs, access to health care, or even supermarkets to residents in certain, often racially determined, areas. The most devastating form of redlining, and the most common use of the term, refers to mortgage discrimination.

Read also the RACIAL REDLINING: A Study of Racial Discrimination by Banks and Mortgage Companies in the United States:

This study examines the issue of racial redlining by major mortgage lenders in the nation’s larger metro areas. Racial redlining is the practice whereby mortgage lenders figuratively draw a red line around minority neighborhoods and refuse to make mortgage loans available inside the red lined area. Broadly defined, racial redlining encompasses not only the direct refusal to lend in minority neighborhoods, but also procedures that discourage the submission of mortgage loan applications from minority areas, and marketing policies that exclude such areas.
In direct economic terms, racial redlining reduces housing finance options for borrowers in minority neighborhoods and weakens competition in the mortgage market. This often results in higher mortgage costs and less favorable mortgage loan terms. More subtly, racial redlining discourages minorities from pursuing home ownership opportunities and in the broadest sense further entrenches the debilitating sociological effects of racial discrimination.

Especially read the section, Worst case lending patterns to see what unregulated redlining produces.