You may have heard about the Overton window, and that’s not about to stop. With the political landscape shifting in sometimes startling ways, what was once an obscure idea has gained broader relevance.

But while the term has been bandied about lately, it hasn’t always been by people who know what they’re talking about. And it’s important to get this right. You’ve probably noticed that policies once dismissed out of hand — from “Medicare for all” to a 70 percent top tax rate; from sweeping action on climate change to abolishing Immigration and Customs Enforcement — are being discussed in mainstream circles now. The Overton window is a useful way to understand what’s happening.

Joseph P. Overton introduced the concept in the 1990s as an executive at the Mackinac Center for Public Policy, a conservative think tank in Michigan. He never expected it to gain widespread recognition, said Joseph G. Lehman, president of the Mackinac Center, and it didn’t until after Overton died in 2003.

Overton just wanted to explain to potential donors what the point of a think tank was, so he created a brochure with a cardboard slider. The brochure listed the range of possible policies on a single issue, from least to most government intervention. On education — an example the Mackinac Center uses — it might run from zero public investment in education to compulsory indoctrination in government schools. But neither of those extremes is going to happen. Only part of the range is achievable, and when Overton moved his slider, different policies fell into what he called the window of political possibility.

“Public officials cannot enact any policy they please like they’re ordering dessert from a menu,” Lehman said in an interview. “They have to choose from among policies that are politically acceptable at the time. And we believe the Overton window defines that range of ideas.”

Grassroots mobilization can shift the window. So can think tanks, which was Overton’s point. But despite a misconception driven by Glenn Beck’s novel “The Overton Window,” the window is a description, not a tactic: Shifting it doesn’t mean proposing extreme ideas to make somewhat less extreme ideas seem reasonable.

“It just explains how ideas come in and out of fashion, the same way that gravity explains why something falls to the earth,” Lehman said. “I can use gravity to drop an anvil on your head, but that would be wrong. I could also use gravity to throw you a life preserver; that would be good.”

The key is that shifts begin with the public. Overton argued that the role of organizations like his own was not to lobby politicians to support policies outside the window, but to convince voters that policies outside the window should be in it. If they are successful, an idea derided as unthinkable can become so inevitable that it’s hard to believe it was ever otherwise.

The current shift toward progressive economic policies is clear and quantifiable. Take some of the legislation introduced by Sen. Bernie Sanders, whose 2016 presidential campaign helped popularize these ideas. In 2015, his bills to make public colleges free and expand Social Security had no co-sponsors in the Senate. Two years later, they had seven and 17, respectively, in addition to 50 and 133 co-sponsors in the House. His signature measure, the Medicare for All Act, had no Senate co-sponsors in 2013 (he didn’t introduce it in 2015), but four years later it had 16, along with 125 in the House.

“We have come a very, very long way in the American people now demanding legislation and concepts that just a few years ago were thought to be very radical,” Sanders said in a recent interview.

His support for these policies set him apart in the 2016 Democratic field, but they are mainstream positions among the 2020 candidates — because, increasingly, they are mainstream positions among the voters those candidates are courting. Sanders emphasized as much in announcing his second presidential campaign last Tuesday.

Most telling, perhaps, is that even opponents are taking the ideas seriously: They might not want Medicare for all, but they believe it could happen and are fighting it accordingly. If a policy is dead on arrival, you don’t have to fight it.

That the Overton window is shifting doesn’t necessarily mean policies like Medicare for all will be enacted, and it doesn’t say anything about whether they are good or bad. But it does say something meaningful about the political climate.

Part of the story is polarization: Democrats moving left and Republicans right, to an extent “that we haven’t seen previously in a modern political period,” said Mary Layton Atkinson, a political scientist at the University of North Carolina at Charlotte who studies public opinion and issue framing. “Republicans have become just as entrenched in their own conservative policy preferences.”

As support for more ambitious policies has increased among Democrats, there has also been “a wave of young party leaders who are less encumbered by a long voting history tying them to more moderate and less progressive policy stances,” Atkinson said. “And they’re being supported by a base that is ready to hear these messages.”

But polarization isn’t the only factor. Polls show that some support crosses the partisan divide. Forty-five percent of Republicans in one poll supported Rep. Alexandria Ocasio-Cortez’s suggestion to tax income over $10 million at 70 percent; among all American adults, 59 percent supported that. Thirty-seven percent of Republicans said they would vote for a candidate who supported a Medicare for all plan; 53 percent of all Americans said the same.

Leaders like Sanders and Ocasio-Cortez argue that voters are rejecting long-standing economic assumptions because those assumptions haven’t yielded the promised results. “I think the line of trickle-down economics improving the lives of everybody doesn’t work when in the last 30 or 40 years, the lives of the middle class have become significantly more difficult at the same time as we’ve seen massive income and wealth inequality,” Sanders said.

That sentiment is far from universal, and many Americans still support “trickle-down” policies. Conservatives and some moderates — including possible presidential candidates like Michael Bloomberg — view proposals like “Medicare for all” or a wealth tax as extreme, and it is not clear how those proposals would play in a presidential general election.

But since Sanders and Sen. Elizabeth Warren of Massachusetts, who is also running for president, began speaking on national stages about ideas once confined to small circles, criticism of those ideas has ceased to dominate the political conversation, and voters are seeing practicality in ideas long considered idealistic. (Warren’s campaign did not make her available for an interview.)

“I think people like Warren and Sanders deserve a lot of credit for advancing these ideas before they were cool,” said Tom Perriello, executive director for U.S. programs at the Open Society Foundations, who co-wrote an article last year about the increasing popularity of once-unthinkable policies. “It created a conversation people hadn’t heard before, and then had the option to look at it and say, ‘Wait, that sounds like a much better idea than what I’ve been hearing before.'”

This article originally appeared in The New York Times.