The federal income tax finds you everywhere. State taxes don’t: California takes up to 13.3% of your income, while Nevada takes nothing. Every state taxes property in some way, but at different levels. But state taxes don’t immediately overburden high-tax state residents as badly as this sounds, because of the State and Local Tax Deduction, fondly abbreviated as SALT.

How It Works:

It’s a below the line deduction, which means taxpayers deduct from their federal tax burden the taxes they pay to their states and municipalities once their taxation rate has been calculated. It covers income, real estate, property taxes and sales taxes. 36 states have a progressive income tax, like the federal one, and eight more have a flat tax. Seven states have no income tax (hi, Texas!).

Why We Have It:

The first reason for why we have it is history: SALT has existed since 1913. The second is fairness: if you’re taxed on the same income at two layers, that’s double taxation. Sometimes it’s characterized as a subsidy for high-tax states, because states get the benefit of charging high taxes without the burden of citizens who bear that sticker shock. The idea is, if you got double taxed, you would move.

SALT is the biggest itemized deduction. The Department of the Treasury’s FY2018 report estimates it’s over a billion per year. And at least one version of the Republican tax reform removes it.

What is this all about? 

First, SALT is a clear example of a distortion: states make different choices about tax rates based on their deductibility. Even though congressional government is technically unified, many Republican representatives come from high-tax states, who crafted their apparatus with the understanding that this deduction existed. The sticker shock of double taxation would likely affect both the residents and the government services.

And second, it’s a showcase in how debates about deductions, and how they will affect middle-class taxpayers, are often useless or disingenuous.

Only 29.6% of taxpayers even itemize their deductions. Most take the standard deduction, which, when combined with the personal exemption, will be $10,400 for single taxpayers in the taxable year 2017. So for this to even matter to you, your itemized deductions need to be more than ten thousand dollars—more, if you’re married. When Nancy Pelosi claimed that 50% of households that benefit from this deduction make less than $100,000, what she said was true, but sort of irrelevant, because that means that 50% of households that take this deduction make more than $100,000 a year—and that’s only about 20% of the population. On the other hand, Paul Ryan’s argument that low-tax states subsidize high-tax states doesn’t account for the fact that the net amount high-tax states send to the feds tends to be higher. Chuck Schumer caught a lot of heat when he criticized eliminating the deduction, because he benefits from it. This is accurate, but not really a gotcha: Chuck Schumer lives in a highly taxed state and he has a high income.

So, everyone is a little wrong.

And this makes sense, because public conversations around tax policy anchor around middle-class taxpayers, who are mostly a square peg when it comes to the round hole of specific deductions. Like any deduction, SALT is regressive, because our system is progressive: you pay more because you earn more. So if your deduction is based on what you would pay, your deduction is also worth more when that number is high. Now, just because rich people disproportionately benefit from a deduction does not mean middle-class people will be completely unharmed by its repeal, but that has more to do with what tax change the repeal of the deduction is going to pay for. If the result is a higher standard deduction, that benefits the middle class. If the result is a lower marginal rate, that may be less valuable, depending on the math. It also depends on how ‘middle-class’ is characterized—in this article, everyone under $200,000 made the cut. Additionally, the Alternative Minimum Tax, or AMT, the secret trapdoor to keep rich people from winning the video game, doesn’t permit a SALT deduction.

The SALT deduction is one of several itemized deductions that are on the chopping block, and therefore are the subject of intense debate despite the fact that 69% of the country doesn’t itemize. The reason for that is one you’ve heard before: tax policy is policy. Our deduction apparatus as it stands currently is a complex, incoherent list of things we think you should spend your money on. Health insurance premiums; home mortgages; child care; replacing things you lost in a fire (but not a fire you started); and so forth. There are arguments to be made that these reverse expenditures are a problematic way to go about doing government spending, but it’s clearly not going away anytime soon.

Everyone wants the tax code to be simpler. No one wants to pay more. This is why repealing deductions is such a politically fraught act, and why even in this one weedy section, there are a stack of conflicting stakeholders, with some suggesting a high-income phaseouts, as opposed to repeal. The philosophical question, I think, is what kind of fairness you want. Do you want fairness in that taxpayers don’t get double-counted on the same income based on where they live? Or do you want fairness in that the federal income tax applies the same to everyone, regardless of the taxation choices their state made?

But the real policy question is: what will this, in combination with everything else, do to my tax return? And that’s a question no one has an answer to yet.