In November, Microsoft, Yahoo and AOL announced an ad partnership, which the trio said would benefit advertisers, agencies and publishers by providing “more efficient access to premium online ad inventory”. The agreements among the companies would enable the ad networks operated by each other to offer the others’ premium non-reserved online display inventory to their respective ad customers (in the U.S.)

Today, that all goes into effect.

“Starting this week, the Microsoft Media Network, AOL’s Advertising.com and Yahoo! Network Plus are leveraging real-time bidding (RTB) to offer advertising opportunities across all three networks’ premium, non-reserved, owned and operated display inventory,” says Microsoft’s Daniel Sheinberg. “This means that the Microsoft Media Network now provides marketers with flexible access to an even larger pool of high-quality inventory, and additional audience reach, beyond our own premium assets. For our advertiser and agency customers, we will continue to provide the high-quality audience they expect, and now we will be able to deliver at greater scale, and with greater ease.”

“This milestone for the Microsoft Media Network is enabled by the advanced technology and capabilities of our Microsoft Advertising Exchange, which we launched in March of 2011,” he continues. “Over the past year, we have been extremely pleased with the Exchange’s performance and the way it’s helped our customers efficiently access and utilize our inventory to build powerful solutions. We’ve seen rapid growth of the Exchange, which is now available in 15 markets globally and continues to ramp.”

“We’re thrilled to partner with Microsoft and AOL and bring to market what we believe will be a more efficient, effective and more effortless way to access true premium inventory and formats,” said Ross Levinsohn, Yahoo! EVP of the Americas, when the partnership was announced four months ago.

Yahoo and AOL actually have an agreement without Microsoft that extends to Canada.