Speculation that Republican presidential candidate Donald Trump went years — maybe decades — without paying federal income taxes has generated questions about "loopholes" available to real estate developers.

Experts say the tax code is indeed riddled with tricky ways to dodge taxes through sophisticated uses of trusts, partnerships, S corporations and so forth. Because Trump refuses to release his tax records, no one can know for sure which strategies he has used to determine his tax burden.

But they also say that one tax rule Trump may have used — the "net operating loss carryforward" — is not a loophole. In fact, they say, it is a commonly used accounting tool that evens out gains and losses to impose taxes more fairly.

Here's the definition: When a business taxpayer has negative income, that loss can be used to get a refund for past taxes paid or get carried forward to reduce future tax bills. Under current law, you can carry back two years and carry forward for 20.

While Congress has shifted the number of carryover years from time to time, the basic idea is an old one. Congress has been allowing net operating loss — NOL — credits since 1918.

"NOLs are a very ordinary part of the U.S. tax code and tax codes throughout the world," said Kyle Pomerleau, director of federal projects at the Tax Foundation, a nonpartisan think tank. The purpose is to "adjust for the somewhat arbitrary decision to tax income over a 365-day period between January and December."

Using a Jan. 1 start date to calculate taxes "may work for some businesses whose profit and loss cycle line up in the calendar year, but it may not work for businesses with much longer cycles," he said.

The Tax Foundation offers this example:

"If a business makes $50 in June but loses $100 in July, we call that a $50 loss. A business that makes $50 in December but loses $100 in January is fundamentally the same thing, but straddles the tax year. Net operating losses (NOLs) allow businesses that lose money in one year and make money in another to smooth those ups and downs."

So if a NOL carryover is not a "loophole," then what is?

Annette Nellen, director of San Jose State University's graduate tax program, said a real estate loophole appears when a tax law is so poorly written that one could avoid taxes that really should be owed.

She offered this example: A local government may tax "agricultural" property at a lower rate than other land. So a property owner might plant five fruit trees and then declare the land "agricultural" just to pay lower taxes than his neighbors.

A loophole means "there is a law, but you could work around it," she said.

The debate over loopholes and carryovers began with a New York Times story, published late Saturday, that revealed documents showing Trump had declared a $916 million loss on his 1995 income tax returns.

The article said that laws in place in 1995 "could have allowed him to legally avoid paying any federal income taxes for up to 18 years."

Nellen said the documents were too limited to show whether Trump used NOL carryforwards or loopholes. She said understanding a developer's tax obligations can be very complicated because "real estate deals typically involve a lot of deductions."

For example, there can be deductions for interest payments, depreciation and much more, she said. Moreover, "we don't know what made up his losses," she said. In other words, unless Trump releases his tax records, his tax payments — or lack thereof — will remain a mystery.

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