WASHINGTON ― One of several changes Senate Republicans made to their tax bill before passing it in the middle of the night last weekend would give a new tax break to oil and gas companies.

The amendment typifies the ad hoc way the tax bill has been written. The measure would ensure a very, very specific type of business benefits from tax reform ― not because this specific category of firm wouldn’t have benefited from the legislation without the amendment, but because of concern that it wouldn’t benefit as much as other companies.

Gregg Polsky, a tax professor at the University of Georgia School of Law, likened it to Oprah Winfrey giving everyone in her audience a car.

“It’s like Oprah for big business,” Polsky said. “You get a car, you get a car, everybody gets a car!”

Like a lot of things in the legislation, the oil and gas amendment, by Sen. John Cornyn (R-Texas), came about as a downstream consequence of Republicans’ most cherished part of their bill: a corporate tax cut that would reduce the top corporate rate from 35 to 20 percent.

Most companies don’t pay the corporate tax, however, with the government instead taxing their income when it passes through to their owners’ individual returns. Such “pass-through” businesses would still benefit from the legislation’s lower individual tax rates and other business changes, but to be extra fair, Republicans want to give them an additional break. The House version of the legislation would create a special 25 percent pass-through rate; the Senate version would allow people who report certain types of business income on their individual returns to reduce their tax liability by deducting 23 percent of that business income from their total taxable income.

Pass-through taxation is already advantageous for businesses because their earnings are only taxed once, when owners and partners pay their individual taxes, whereas corporate earnings are taxed directly at the entity level and again when profits are distributed to individual shareholders. Pass-throughs are not allowed to have more than 100 shareholders, though, so it’s not as easy for them to raise capital.

Cornyn’s amendment benefits a special category of businesses that already get a sweet deal: they are allowed to have thousands of investors, like corporations, but are taxed as pass-throughs. They’re called publicly traded partnerships, or master limited partnerships. They are concentrated in the oil and gas industry and there aren’t very many of them, though their ranks include subsidiaries of notable brands such as Shell and Valero. The Joint Committee on Taxation estimates Cornyn’s amendment, which would specifically allow publicly traded partnerships to claim the pass-through deduction, would cost only $700 million over 10 years. (The deduction overall will cost more than $400 billion.)

“Under current law, publicly traded Master Limited Partnerships are taxed as pass-through entities and this amendment simply preserves that status in the new bill,” a Cornyn spokesman said in an email.

Publicly traded partnerships have existed since the 1980s, according to the Congressional Research Service. After the 1986 tax reform bill reduced individual tax rates relative to corporate rates, giving pass-through businesses an advantage, lawmakers realized they might have allowed too many businesses to escape corporate taxes. They required publicly traded firms to file as corporations ― with an exception for companies deriving 90 percent of their income from certain activities, especially oil and gas extraction.

Publicly traded partnerships have managed to maintain their special status and even expand the types of business activity eligible for the exception, like transporting renewable and alternative fuels. But in recent years, Democrats have complained that renewable energy companies are unable to qualify for the special tax treatment and are therefore at a competitive disadvantage.

The Master Limited Partnership Association, a lobbying group for the industry, said in a statement over the weekend that it was “pleased that the Senate understood the important role MLPs play in, among other things, building the critical energy infrastructure that promotes our nation’s energy security and reduces energy costs for consumers.”

Polsky pointed out if Republicans’ tax reform truly disadvantages pass-through businesses relative to corporations, pass-throughs always have the option of becoming regular “C” corporations.

“You don’t need a pass-through rate at all; if pass-throughs think they can get a better deal by being a corporations, literally all they need to do is check a box and boom, they’re a corporation,” Polsky said in an email.

The Senate bill still has to be merged with the House bill and both chambers must vote on identical legislation before it can be signed into law by President Trump.

Arthur Delaney co-hosts the HuffPost Politics podcast: