Microsoft announced on Tuesday that it is partnering with AOL and Yahoo to sell premium online display inventory.

The agreement will allow each network to offer each other’s premium non reserved online display inventory to advertising customers. Microsoft, Yahoo and AOL will all continue to operate their core networks and advertising separately but the partnership will benefit advertising customers wishing to buy up inventory across all the sites.

“Enhancing choice and scale in today’s display advertising market is ‘a rising tide that lifts all boats’,” said Rik van der Kooi, corporate vice president of the Microsoft Advertising Business Group. “This partnership will create an opportunity where advertisers and publishers alike can benefit from easier access to — and demand for — high-quality inventory. The fact that we’re joining together to offer this kind of access to quality — yet each with our own differentiated ad offerings — is something that will benefit the market as a whole.”

The agreement covers Canada and the United States and will come into effect in 2012. Microsoft says AOL and Yahoo will both integrate their real-time bidding (RTB) technologies to ensure inventory can be sold across all sites. Microsoft Advertising Exchange and Yahoo’s! Right Media Exchange will be the primary sources of sales.