The House and Senate have now passed their respective tax bills. Next, they have to figure out how to reconcile their differences.

The negotiations will happen in a conference committee, where Republicans from the House and Senate will get together to try to hash out a final plan. Once they do, both chambers will have to pass it again. Republican leaders and the White House want to pass a final bill by the end of the year and send it to President Trump’s desk.

The House and Senate bill have their biggest features in common: They would both slash the corporate tax rate from 35 percent to 20 percent, a historic reduction in the taxes levied on businesses at a time when Americans support raising taxes on corporations.

But there are also some big differences, including how permanent this tax overhaul would be. The House bill makes its changes to the individual tax code permanent; the Senate bill would allow many major provisions to expire after 2025 in order to comply with Senate rules that limit how much a bill that can’t be filibustered can increase the federal deficit.

The House and Senate have also structured their tax cuts for “pass-through” businesses, like LLCs or partnerships, completely differently. The pass-through issue almost derailed the Senate tax bill, so it will be an area to watch closely as the chambers work out a new bill.

No issue seems so big that it would derail conference committee proceedings, which almost never fail. But tax policy is fickle, and every percentage point change means millions or billions of dollars being shuffled around the US economy.

Here is what House and Senate Republicans need to work out.

The biggest issues are the federal deficit and “pass-throughs”

If Republicans want to pass this bill on party lines in the Senate, the final bill has to address the deficit.

The tax bill lives and dies by a budget rule known as the Byrd Rule, a condition of the process that allows Republicans to pass legislation that isn’t subject to a filibuster and needs only 51 votes in the Senate. Because of how they set it up, Republicans’ tax bill can only increase the deficit by $1.5 trillion in the first 10 years, and can’t increase the national debt outside that window.

The House’s tax bill doesn’t pass this test. The Senate’s barely does.

Over the past several weeks, Republicans in both chambers have tried to repeal or cap various deductions — some big, some small — to offset the cost of massively cutting the corporate tax rate, increasing the standard deduction, and lowering individual rates. The process has earned the ire of various factions of the party; cutting various deductions is politically difficult work. But neither the House nor the Senate could get the math to work just by changing deductions.

The Senate got around this problem by sunsetting almost all the individual tax relief measures at the end of 2025 — including the increased child tax credit, the doubled standard deduction, the estate tax cut, and even the tax break for pass-through business income. By year 10, the Senate bill doesn’t increase the deficit, according to the Joint Committee on Taxation, suggesting that it won’t raise the deficit over the long run. Some revenue raisers on the individual side, like ending deductions for state and local taxes and the elimination of personal exemptions, would expire at the end of that year too.

In other words, Senate Republicans put a hard stop to all the most expensive parts of the individual tax reforms to pay for a permanent corporate tax cut.

The House sunsets some provisions in its bill — like the $300 family credit, which ends after five years — but it doesn’t go nearly as far as the Senate bill. The Congressional Budget Office estimates that the House bill will increase the deficit past the initial 10-year window, making it untenable under Senate rules.

This is the biggest difference between the House and Senate bills — and poses one of the biggest obstacles to putting the two bills together. Many House Republicans were promised a better bill once the Senate passed its tax proposal and the two chambers went to conference.

This will affect how the two chambers reconcile other major differences, like how they tax pass-through businesses, which aren’t taxed like corporations and instead pay taxes on business income as if it were personal income.

The House bill gives pass-through businesses a reduced rate at 25 percent. The Senate bill addresses pass-throughs differently, allowing them to deduct 23 percent from their taxes, in addition to lowering the top tax rate from 39.5 percent to 38.5 percent.

It will also impact how much other deductions can be changed.

The House and Senate have a lot of smaller differences too

The deficit and pass-throughs seem like the issues most likely to trip up the conference committee, because they were the biggest problems in the Senate. But there are a lot of other differences that the House and the Senate will have to resolve in the next few weeks, including:

Individual tax rates: The House bill reduces the number of tax brackets to four — 12 percent, 25 percent, 35 percent, and 39.6 percent for the top earners. The Senate keeps seven as currently exist: 10 percent, 12 percent, 22 percent, 24 percent, 32 percent, 35 percent, and a lowered top rate of 38.5 percent.

The House bill reduces the number of tax brackets to four — 12 percent, 25 percent, 35 percent, and 39.6 percent for the top earners. The Senate keeps seven as currently exist: 10 percent, 12 percent, 22 percent, 24 percent, 32 percent, 35 percent, and a lowered top rate of 38.5 percent. Child tax credit: The House bill increases the credit to $1,600. The Senate bill increases it to $2,000.

The House bill increases the credit to $1,600. The Senate bill increases it to $2,000. Mortgage interest deduction: The House bill caps the deduction for home values up to $500,000 for new purchases. The Senate bill keeps it at $1 million.

The House bill caps the deduction for home values up to $500,000 for new purchases. The Senate bill keeps it at $1 million. Medical expense deduction: Current tax law allows Americans to deduct qualified medical expenses that cost more than 10 percent of their adjusted gross income. This can include surgeries, ongoing treatment, preventive care, and dental and vision expenses, and is a deduction typically used by people with chronic illnesses. The House bill repeals it. The Senate bill keeps it and actually expands it.

Current tax law allows Americans to deduct qualified medical expenses that cost more than 10 percent of their adjusted gross income. This can include surgeries, ongoing treatment, preventive care, and dental and vision expenses, and is a deduction typically used by people with chronic illnesses. The House bill repeals it. The Senate bill keeps it and actually expands it. Education deductions: Graduate students staged walkouts across the country this week over the House bill’s provision to repeal deductions for student loans and other education expenses like teachers buying school supplies. But Senate bill would keep these deductions intact.

Graduate students staged walkouts across the country this week over the House bill’s provision to repeal deductions for student loans and other education expenses like teachers buying school supplies. But Senate bill would keep these deductions intact. Corporate tax cut start date: Both bills cut the corporate rate from the current 35 percent to 20 percent, but the House bill cuts it immediately while the Senate bill delays the cut until 2019.

Both bills cut the corporate rate from the current 35 percent to 20 percent, but the House bill cuts it immediately while the Senate bill delays the cut until 2019. Estate tax: The House bill eventually repeals it entirely. The Senate bill only expands how much of an inheritance can be exempt from it and then allows that exemption to expire.

The House bill eventually repeals it entirely. The Senate bill only expands how much of an inheritance can be exempt from it and then allows that exemption to expire. Obamacare’s individual mandate: The House bill doesn’t touch it. The Senate bill repeals it. An estimated 13 million fewer Americans would have health insurance if it were repealed.

That’s a lot of fine print to work out. Every change involves millions or billions of dollars. The individual mandate in particular is a lot of money: $330 billion, according to the Congressional Budget Office, because fewer people would have insurance and there would be less federal spending on Medicaid and insurance subsidies.

The Senate will likely have a stronger hand in these negotiations: It’s the upper chamber that must navigate the byzantine “budget reconciliation” rules and a smaller margin for error.

But the House is insisting that it won’t simply yield to all the Senate’s demands. It promises to be a dramatic December, with an end-of-year deadline looming and a volatile president who could insert himself into the talks at any moment.