Leaders at the Group of 20 summit in Rome are expressing broad support for sweeping changes in how big global companies are taxed.

The goal: deterring multinationals from stashing profits in countries where they pay little or no taxes — commonly known as tax havens.

The proposal was finalized in October among 136 countries and sent to the G-20 for a final look after complex talks overseen by the Organization for Economic Cooperation and Development. It would update a century’s worth of international taxation rules to cope with changes brought by digitalization and globalization.

Also read: G20: Prime Minister Narendra Modi invites Pope Francis to visit India

Treasury Secretary Janet Yellen says it will end a decades long “race to the bottom” that has seen corporate tax rates fall as tax havens sought to attract businesses that used clever accounting to take advantage of low rates in countries where they had little real activity.

Here’s a look at key aspects of the tax deal:

What was the problem?

In today’s economy, multinationals can earn big profits from things like trademarks and intellectual property that are easier than factories to move. Companies can assign the earnings they generate to a subsidiary in a country where tax rates are very low.

Also read: G-20 endorses global corporate minimum tax at Rome summit

Some countries compete for revenue by using rock-bottom rates to lure companies, attracting huge tax bases that generate large revenue even with tax rates only marginally above zero. Between 1985 and 2018, the global average corporate headline rate fell from 49% to 24%. By 2016, over half of all U.S. corporate profits were booked in seven tax havens: Bermuda, the Cayman Islands, Ireland, Luxembourg, the Netherlands, Singapore and Switzerland. White House officials are saying the global minimum would result in almost $60 billion of added U.S. tax revenue.

How would a global minimum tax work?

The basic idea is simple: Countries would legislate a minimum rate of at least 15% for very big companies with annual revenues over 750 million euros ($864 million)

If company earnings go untaxed or lightly taxed in one of the world’s tax havens, their home country would impose a top-up tax that would bring the rate to 15%.

Also read: G20: Biden receives Communion in Rome amid debate on his abortion views

That would make it pointless for a company to use tax havens, since taxes avoided in the haven would be collected at home.

What is the US role in the agreement?

Biden has staked a claim that the U.S. must join the global minimum tax in order to persuade other nations to do so. That would involve raising the current rate for foreign earnings from 10.5% to reflect the global minimum. His tax proposals are still being negotiated in Congress.

U.S. participation in the minimum tax deal is crucial, simply because so many multinationals are headquartered there — 28% of the 2,000 biggest global companies. Complete rejection of Biden’s global minimum proposal would seriously undermine the international deal.

Also read: US, EU agree on a deal to settle rift over Donald Trump-era trade tariffs

Does everyone like the deal?

Some developing countries and advocacy groups such as Oxfam and the UK-based Tax Justice Network say the 15% rate is too low. And although the global minimum would capture some $150 billion in new revenue for governments, most of it would go to rich countries because they are where many of the biggest multinationals are headquartered. Developing countries took part in the talks and all signed except for Nigeria, Kenya, Pakistan and Sri Lanka.

U.S. critics including Republican leaders and some business groups say the proposed minimum tax would make America less competitive and potentially cost jobs, a sign that the key is to get passage from other nations so the U.S. is not disadvantaged.