The fair-housing case that caught the Supreme Court's attention this term turned on the specific question of how the Texas Department of Housing and Community Affairs has allocated tax credits for affordable housing.

The formula, claimed the nonprofit suing the agency, effectively guaranteed that affordable housing in Dallas was concentrated in poor, inner-city minority neighborhoods, and was hard to find in the city's white suburbs. As a result, the lawsuit argued, the housing agency denied low-income minorities the chance to live in neighborhoods with better schools and greater opportunity, perpetuating decades-old patterns of segregation.

The Supreme Court, in a 5-4 decision, ruled that cases like this can be brought under the Fair Housing Act, even if the officials who wrote that formula didn't intend to discriminate. The Fair Housing Act, the court reaffirmed today, prohibits not just intentional discrimination, but also policies that can have a "disparate impact" on minorities.

This is not, on the one hand, a groundbreaking reading of the law. Lower courts, civil rights lawyers and federal agencies have interpreted the Fair Housing Act this way for decades, even though the 1968 law didn't explicitly contain the words "disparate impact." On the other hand, the ruling is momentous: Had a single justice — likely Anthony Kennedy, who wrote the opinion — felt differently, another key piece of civil rights law in the U.S. would have been substantially weakened.

The ruling is also significant because it implicates policies far beyond formulas that dictate where affordable housing tax credits are used, and these policies exist in communities all over the country.

As Kennedy wrote in the decision:

These unlawful practices include zoning laws and other housing restrictions that function unfairly to exclude minorities from certain neighborhoods without any sufficient justification. Suits targeting such practices reside at the heartland of disparate-impact liability.

Such zoning laws and restrictions take many forms that look like they have nothing to do with race. They exist in local laws that say residences must be built on a minimum lot size of an acre or more, a requirement that effectively keeps out denser row homes or multi-family buildings that would be more affordable than large-lot homes. They exist in laws allowing only one residence per lot, which block apartments (and the lower-income residents who might live in them).

They exist in laws capping the share of properties in a community that can be rented — or dictating that homeowners can rent only to their "blood relatives." They exist within housing agencies that funnel affordable housing into poor neighborhoods on the thinking that we should offer such housing to poor people where they already live.

Policies and ordinances like this are commonplace. The communities that adopt them don't usually announce that they're trying to keep out poorer families or segregate minorities, and they may not even mean to.

"The idea of disparate impact," says Betsy Julian, the president of the Inclusive Communities Project that brought the case before the Supreme Court, "is if what you do has the same impact as what somebody would do if they intended to discriminate, then disparate impact means you’re still responsible for that harmful impact, unless you have a a good justification."

Kennedy was strikingly clear that exclusionary zoning lies at the "heartland" of that liability.