Quick, how many houses are there in the US Congress? (Decoder admits that this is a trick question.)

Yes, there’s the House and Senate. That’s two.

But on Capitol Hill, many say there are three. The House, the Senate, and the conference committee.

Conference committees are groups of senators and congressmen who get together to resolve differences between their chambers on a major bill. And before you nod off from boredom, let Decoder make this point: Healthcare reform legislation will have to pass through a conference committee if it is to become law. And that is when some of the most important decisions about the bill’s structure might be made.

“For a big bill like that, it could be the critical stage,” says political scientist Ross Baker, an expert on congressional process at Rutgers University in New Jersey.

Conference committees are like Narnia for legislators. They are magical places where many of the normal rules are suspended and where scrutiny, whether from parents or press, is less than normal.

No, they are not reached by walking through the coats in the House or Senate cloakrooms.

They exist because the House and Senate must pass final bills with the exact same wording. Yet both chambers generally produce their own preliminary versions of major legislation.

Conference committees convene to reconcile wording, dropping a House provision there, adding a Senate provision here. Generally, they split the difference on allocated funds. Stuff like that.

Members are appointed by the speaker of the House and the Senate majority leader. Customarily, the chairmen of committees with an interest in the bill get to join. But there are no hard rules on this.

Meetings are supposed to be open, but lots of bargaining goes on behind closed doors.

Members aren’t supposed to add things that aren’t in either the House or Senate versions of the bill, but it still happens.

“Conferences are marvelous. They’re mystical. They’re alchemy. It’s absolutely dazzling what you can do,” according to Alan Simpson, a former GOP senator from Wyoming.

In a 2005 conference on an emergency supplemental bill, for instance, conferees added $4 million to pay off the debt of the Fire Services Academy and put in a provision that would have allowed drilling for natural gas in a national park.

Right now, there are big differences between the House and Senate versions of healthcare reform. The conference to resolve those could be as contentious as any in years.