A new mapping tool from the National Community Reinvestment Coalition illuminates the consequences of the most important history underlying cities in the United States today: the history of redlining. The tool layers data on current income status and minority population on top of maps created using data from the historical redlining maps that were digitized earlier this year by researchers at the University of Richmond.

Redlining set the stage for many of the familiar stories about “inner city” neighborhoods that we know today. Let’s pick just one: gentrification.

Starting in the 1930s, federal housing policies in the U.S. spurred lenders to make loans almost exclusively to white households to buy homes in exclusively white neighborhoods. Predominantly minority communities, meanwhile, were designated too risky for loans. (The red lines that federal agencies often drew on maps to designate neighborhoods of color gave rise to the term “redlining.”) The people in redlined neighborhoods and their landlords both were denied the capital needed to maintain and modernize buildings. Many buildings gradually fell into disrepair.

Private equity investors turned their gaze in earnest to cities in the early 2000s. Broadly speaking, they aimed to buy low and sell high. While they may or may not have known that redlining was why certain neighborhoods were low in value, investors started seeking out low-value neighborhoods, snapping up properties, kicking out existing tenants and flipping them into luxury condos or rentals. Even rent-stabilized apartments aren’t safe from this business model.

Now, pick almost any hot neighborhood today — and you are likely to find a formerly redlined area that is now being transformed.

Brooklyn’s Williamsburg (dots cover areas where 2015 median family income is below 80 percent of area median income; dashes outline areas that were at least 50 percent minority in 2010):

Philadelphia’s Northern Liberties:

Pittsburgh’s East Liberty:

San Francisco’s Mission District:

Seattle’s Central District:

While each neighborhood and each city has a lot of unique aspects that can’t be ignored, they all also share a certain history that is often ignored. Federal housing policy, not to mention racial discrimination in lending, forced people of color to live in certain neighborhoods; those neighborhoods were subsequently allowed to decline in value due to limited access to capital, and today those neighborhoods are now the hottest targets for investors.

You could say drugs played a role in neighborhood decline. You could say violent crime played a role in neighborhood decline. You could say mass incarceration or the loss of industrial jobs played roles in neighborhood decline. But NCRC’s redlining map layers today’s income and race data on top of maps made long before any of that happened, illuminating a deeper history of how federal policy artificially lowered the value of certain neighborhoods to begin with, making them more vulnerable to all of the above.