Provisions in the U.S. Treasury’s ‘Green Book’ proposal could disrupt or dismantle some of the most popular ways the ultra wealthy have legally avoided taxes for decades.

One target is dynasty trusts, vehicles that wealthy families can use to benefit multiple generations of descendants. Another is an even more common tool among the top 0.1% — trusts that can move millions, and sometimes billions, of dollars to heirs tax-free. The proposed plan would also charge a capital gains tax when assets are transferred into or distributed from certain kinds of trusts.

Advisers interpreting the Green Book can’t be sure exactly how the provisions will work in practice. The final language in any tax bill will be up to Congress and some details may not be settled until the IRS issues regulations.

"The underlying feeling at the client level is uncertainty and fear because of the lack of predictability about what the tax laws hold,” said Megan Jones, an Estates, Trusts & Tax Planning lawyer at Pillsbury. “I’ve used the technical term ‘freaking out," she joked. “Clients have a lot more questions and a lot more concerns.”

Read more in Bloomberg.