Support for NPR and the following message come from SAP Concur, a leading brand for integrated travel expense and invoice management solutions.

With SAP Concur solutions, you'll be ready to take on whatever the market throws at you next. Learn more at concur.com.

This is Planet Money from NPR.

There is a kind of tax that a lot of people have imagined and dreamed of really for years now.

A wealth tax.

A way to tax people not just on what they earn, but on what they own.

Houses, stocks, Picasso's, superyots, thoroughbred horses, whatever.

Supporters of this idea tend to be pretty progressive and they say a wealth tax would be this

powerful way of fighting inequality but the legality of a wealth tax that has been in question for a long time and this

term the Supreme Court had a chance to weigh in the case that gave him that chance

it's called Moore versus the United States.

So the Moors are an American couple

named Charles and Kathleen Moore.

And in the mid-2000s, they invested

in this business in India that provides equipment

to low-income farmers.

Over time, this business grew, it made profits, and the profits got reinvested in the company.

Now, usually when that happens, the company's investors, they can defer taxes on those profits if they're made outside

the US.

But in 2017, Congress passed a new law that said for foreign corporations, Americans like the Moors would have to pay a one-time tax on those profits,

even if the profits never actually reached their own pockets.

The Moors argued they were being taxed on what investors call

...