Update 8:20 p.m: Late Thursday, Health and Human Services Secretary Kathleen Sebelius extended the deadline until Dec. 14 for states to decide whether to run an exchange on their own.

Come Friday, states will have to decide whether they will run their own insurance exchanges under President Obama's sweeping health law.

These exchanges will be where people and small businesses go to shop for insurance.

The policies offered will have to meet minimum standards. And there will be subsidies on a sliding scale for people who need them. Those will be available for people earning up to four times the federal poverty level, or about is about $40,000 to $45,000 for a single person.

At noon Thursday, we'll convene a Twitter chat about the exchanges: why they matter and how they could transform the health insurance marketplace.

Look for the hashtag #healthexchange

Our lineup includes:

NPR's Julie Rovner, who put together this explainer on the exchanges.

Colorado Public Radio's Eric Whitney, who recently reported on the state's early decision to build an exchange.

Health economist Austin Frakt, part of the brain trust at the Incidental Economist blog, will chime in.

Avalere Health's Caroline Pearson's will bring her insights to the party.

And NPR's Richard Knox will help fill us on what's happened with the pioneering exchange in Massachusetts.

Update 3:45 p.m.: David Schultz pulled together highlights from the chat.

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