In recent years, the term “redlining” has become shorthand for many types of historic race-based exclusionary tactics in real estate — from racial steering by real estate agents (directing Black home buyers and renters to certain neighborhoods or buildings and away from others) to racial covenants in many suburbs and developments (barring Black residents from buying homes). All of which contributed to the racial segregation that shaped the way America looks today.

But what was redlining, really?

The origins of the term come from government homeownership programs that were created as part of the 1930s-era New Deal. The programs offered government-insured mortgages for homeowners — a form of federal aid designed to stave off a massive wave of foreclosures in the wake of the Depression.

As these programs evolved, the government added parameters for appraising and vetting properties and homeowners who would qualify. They used color-coded maps ranking the loan worthiness of neighborhoods in more than 200 cities and towns across the United States.

Neighborhoods were ranked from least risky to most risky — or from “A” through “D.” The federal government deemed “D” areas as places where property values were most likely to go down and the areas were marked in red — a sign that these neighborhoods were not worthy of inclusion in homeownership and lending programs. Not coincidentally, most of the “D” areas were neighborhoods where Black residents lived.

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In 1976, the historian Kenneth T. Jackson discovered one of these government maps of St. Louis. “When Jackson discovered this map, it was the smoking gun,” said Matthew Lasner, an associate professor of urban studies and planning at Hunter College. (Mr. Jackson says he discovered the map somewhat by accident while searching for other housing records.)