In a Washington Post interview Tuesday, Sen. Rand Paul (R-Ky.) said, “a flat tax is progressive. I don’t know how many people realize this, but 10 percent of a million is more than 10 percent of a thousand.”

He’s right about the math. But “progressive”? In the tax world, that means something — and definitely not what Paul says.

As most people who follow taxes know, a “progressive” tax means people pay a higher percentage of income as they get richer. This is how American taxes work now—you might only pay 10 percent of income up to $9,000, but after that you’re in a higher bracket and pay more of each dollar.

A flat tax would wipe out those brackets. In fact, the whole point is that it’s not progressive. It’s not regressive, either. It’s just flat.

This isn’t Paul’s first blunder on taxes: he seemingly underestimated the costs of his own tax plan by $1 trillion recently. But in this case, there’s a strange catch. Paul’s own tax plan is progressive, because it’s not a flat tax in its purest form. He only wants one bracket — a 14.5 percent rate for everyone – but he includes a $15,000 standard deduction and $5,000-per-person personal exemption. With these breaks, a family of four would pay no taxes on its first $50,000 in income. Effectively, this creates two tax brackets: one at 0 percent and one at 14.5 percent. So poorer people do pay a lower rate. (A pure flat tax, in which every American suddenly pays one tax rate, would be a huge hit for the poor, who often don’t pay taxes at all.)

Paul isn’t alone here: A number of other GOP candidates have pledged their loyalty to the flat tax as well and many on the right consider it the gold standard of tax proposals. But no GOP plan, past or present, is a pure flat tax.

“Every flat tax actually introduced or proposed … has had large exemptions,” David Burton, a tax expert with the Heritage Foundation, wrote in an email.

So why this focus on “flatness,” if nobody actually wants a truly flat tax? The reason is a window into the real politics of taxes, which can be very different from the way candidates talk about them.

When Paul attacks American taxes, what he’s really attacking is their bewildering complexity – in a video, he theatrically dices up the tax code with a chainsaw. A flat tax, like Paul’s, does something that a lot of Americans would be grateful for: it throws out the Byzantine system of loopholes that make paying taxes such a grueling, time-consuming exercise.

Rhetorically, you can understand why the ‘flat tax’ has garnered so much support. But its name, and the focus on fairness and simplicity, tends to obscure what’s happening under the hood. Every change in taxes is actually a decision to benefit one group or the other. In the current loophole-filled tax code that politicians love to hate, those loopholes tend to benefit the rich. And to compensate for erasing them, Paul’s flat tax lowers the rate on the wealthy so much that they end up being the overwhelming beneficiaries. According to the Tax Foundation, people earning less than $100,000 would receive, at most, a 3 percent tax cut from Paul’s plan. People making more than $1 million would receive a 13 percent tax cut.

Paul could have created an equally fair and simple tax code, but with more tax brackets. Or he could have focused more on what it would actually do to the system: the sudden erasure of every loophole and special exemption in America’s tax code, with a big bucket of tax relief for the rich. Of course, both of those are a lot harder to say than “flat.”

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