President-elect Donald Trump will be audited every year during his term due to a provision in the IRS manual that dates back to Richard Nixon’s presidency.

Both the president and vice president have been under a mandatory annual audit by the IRS since the Watergate era of the 1970s.

The little known provision is an internal requirement, meaning it does not come from a law that mandated a corresponding regulation, McClatchy reported. Trump could presumably attempt to challenge or change the practice. 

“I don’t believe there is anything that would prevent the president from … instructing that this precautionary measure of the IRS audits be rescinded,” Norman Eisen, a former special counsel to the Obama White House told McClatchy. “He could theoretically do it.”

Every president since Nixon has released tax returns, but Trump could choose not to and cite his international business interests as a reason to keep them private.

If Trump chooses to not release his returns, the only ways the public would know how much he pays in taxes is if there is an unlawful leak, or if he faced criminal prosecution for tax fraud. 

“The returns are protected by law,” Joe Thorndike, a tax historian who runs the website taxhistory.org, told McClatchy. “He’s not required to release anything so we have to rely on the agency.”

Nixon’s vice president, Spiro Agnew, resigned in 1973 after pleading guilty to tax evasion. Nixon settled with the IRS in 1974 and paid $465,000 in back taxes after not paying any for more than three years.