Wikimedia Commons Microsoft, AOL, and Yahoo have agreed to begin selling remnant, or "class two," inventory on each other's sites, Peter Kafka reports.

"The theory," says Kafka, "is that if, say, AOL has a big order for a certain kind of ad impressions, it will fill it with its own inventory as well as what’s available from Microsoft and Yahoo."

The companies will split the revenues on any sales made this way.

The goal: win back some of the ad dollars flowing to ad networks, Facebook, and Google.

Executives from the three companies briefed Web publishers and ad-buyers on the deal at a dinner last night.

Yahoo EVP Ross Levinsohn, AOL's Ned Brody and Tim Armstrong, and Microsoft ad COO David O'Hara repped their companies.

It is a non-exclusive deal. It will start before the end of this year.

Go read Kafka's detailed report >>

Related: How Do AOL Ad Sales People Like The Idea Of Yahoos Selling Their Inventory?