Donald Trump has announced that he will not release his tax returns before the presidential election. That controversial decision has elicited a challenge from fellow billionaire and Hillary Clinton supporter Warren Buffett, who’s offered to hold a joint-forum in which the Oracle of Omaha and the developer-candidate both field questions on their filings from an audience of voters.

Given all the mystery and conflicting claims surrounding Trump’s reluctance to comply with what’s standard practice for presidential candidates, it’s instructive to review the main features of the Trump tax controversy. Here are the big questions and Fortune’s answers, largely drawn from our coverage of Trump’s finances over the past several months.

Is It Unusual for a Presidential Candidate to Not Release Personal Tax Returns?

Absolutely. Every major party candidate since Richard Nixon in 1972 has made their returns public before election day. Only one truly competitive candidate refused, and that was billionaire Ross Perot, who won 19% of the vote as an independent in 1992. In most cases, candidates make their filings public around tax time in April, before their conventions and several months prior to the election. The only Republican or Democratic candidate to release afterthe convention was Mitt Romney in 2012. The delay allowed opponents to wrongly claim that Romney paid no taxes, and the candidate paid dearly for the delay. Trump recently stated that Romney suffered even more by releasing them, and “might have lost the election” because of the way Democrats distorted the details they disclosed.

“If you remember Harry Reid lied about it, he told a dirty lie,” Trump said on Fox News last month. “And Mitt gave that and after he gave it, they found a little sentence and they made such a big deal. He might have lost the election over that.”

So far, Trump’s supporters seem unfazed by his refusal to follow longstanding tradition, but Hillary Clinton is exploiting the issue, claiming that Trump may be concealing that he donates nothing to charity and may be concealing questionable financial dealings.

What’s Trump’s Reason for Refusing to Follow Longstanding Practice?

Ever since the Republican debates, Trump has stated that several years of his returns are under an IRS audit, and that he won’t release them until the audit is completed. In late July, his campaign said definitively that Trump will not make his filings public before the election, once again citing the audit, which the Trump campaign expects to extend beyond November 8. Trump, however, also pledged to make his returns public if he’s elected president, following the conclusion of the audit.

What Has Trump Said About His Taxes, and What Are They Likely to Reveal?

In a television interview in May, Trump told ABC’s George Stephanopoulos that his tax rate is “None of your business,” and “I fight very hard to pay as little tax as possible.” That statement led to widespread speculation that Trump pays little or no personal income tax.

If and when they’re released, Trump’s returns would provide at least two crucial pieces of information:

First, how much money does he donate to charity? Charitable giving is traditionally considered a great point of pride—and a civic responsibility—by presidential candidates. The perception that Trump is uniquely ungenerous could unsettle voters.

Second, we’d learn how big a check, if any, Trump writes each year to the U.S. Treasury. Most voters don’t expect to receive protection from the military for free, or demand that others to pay for the Department of Education, Medicare transfers to the states, and all other federally-funded programs. If Trump pays little or no tax, the tens of millions who pay income taxes are effectively paying his way.

How Can Someone Who Claims To Be Worth $10 Billion Pay Little Or No Tax?

Trump’s returns won’t tell us how rich he is, for a basic reason: They won’t show his total revenue, or the amount of cash he puts in his pocket every year. They’ll show his taxable income, calculated after all deductions, some expenses paid in cash, others “accrual items” that offset other income.

In fact, Trump may have properties in what’s known as C-corporations that file 1120 forms with the IRS. A C-corp pays tax on its profits, just like IBM or GM, but the earnings wouldn’t flow to Trump’s personal return unless they pay him dividends. Otherwise, they’d remain as retained earnings in the C-corp. The earnings on buildings or golf courses held in C-corps wouldn’t even show up on Trump’s 1040, the return that he, and all U.S. taxpayers, are obligated to file.

We don’t know for sure that Trump’s tax bill is minuscule, though that’s what his public statements imply. Real estate developers have plenty of ways to avoid taxes, via totally legitimate deductions not available to folks who labor for a company.

That said, Trump must have enormous deductions to erase the pre-tax income that Fortune estimates he earns, not to mention the gigantic numbers the tycoon is claiming. From our analysis of his publicly released balance sheet and filings with the Federal Election Commission, it appears that Trump’s income from rents, royalties, condo sales and the like is at most $160 million.

In a conversation with this writer, Trump stated thatFortune was wildly underestimating the true number, and this is was “about four times that amount,” or over $600 million.

Let’s assume for now that the number is really around $160 million, and once again, that’s an optimistic estimate. Trump’s biggest deductions would be interest expense on his approximately $1 billion in total debt, and depreciation on his investment in buildings and golf courses. Fortunealready deducted the estimated interest expense to arrive at the $160 million. It’s also been widely reported that Trump has invested many hundreds of millions of dollars in his approximately one-dozen golf courses. It’s possible that his depreciation on the golf courses far exceed their income—not unusual in the industry—by a number so big that it also offsets his income from big rental properties such as 40 Wall Street and his 30% holding in the Bank of America complex in San Francisco, in partnership with Vornado Realty Trust.

Beyond the Returns: What Voters Need to Know

Experts say that for the business owners, tax abuse comes in the form of living off their business. It’s highly tempting for ultra-wealthy entrepreneurs who own real estate or other companies to charge their expenses for personal residences, vacation trips, parties, golf fees and sundry extras to their corporations and partnerships. The law requires that the moguls either reimburse a venture they own for payments for any services or properties they use personally, or report those perks as personal income—and pay tax on them.

If an entrepreneur charges a $1 million personal expense to a C-corp, it would lower the C-corp’s taxable income by that amount, and deprive the Treasury of $300,000 that the business person should have paid by reporting the perk as personal income. So the rest of us are effectively paying $300,000 to support our government that the tycoon is skirting, money that’s going to subsidize a gilded lifestyle.

Nominees for ambassadorships and cabinet positions are grilled during background checks by IRS agents to ensure that they’re not disguising personal expenses as tax-deductible business costs. But presidential candidates don’t get the same scrutiny. And they should.

There’s no evidence that Trump is charging any of the almost 100 businesses listed in his filing with the Federal Election Commission for personal expenses. Still, he’s a highly unusual candidate in that he owns multiple residences, including a vacation home at Mar-a-Lago, a Boeing 757-200 that he uses for business and pleasure, and around a dozen golf courses in the U.S. and Europe. In 1989, Leona Helmsley, a fellow Manhattan real estate billionaire, went to jail for billing renovations on her weekend home to her company, and failing to pay taxes on the benefits.

Even if Trump were to release his tax returns, they wouldn’t address the personal vs. business expense issue. Hence, voters who value full disclosure would like to see not just his returns, but lots of additional information demonstrating that the two categories are kept scrupulously separate. Given his immense wealth, Trump should be providing a lot more information than previous candidates, not less.

Trump has often stated that his returns are “all very beautiful,” as if written by monks in gold leaf. It’s highly possible that Trump is scrupulously honest in matters of taxes, a beauty that could be revealed if he lifts the veil. But he’s apparently betting voters to remain curiously uncurious. The question is: Why is he making that bet at all?

This article originally appeared on Fortune.com

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