LAS VEGAS—Microsoft is beginning to articulate a concrete plan for turning Windows Live into a developer platform.

Here at Microsoft's Mix 06 conference on March 20, Microsoft officials explained in a session on Windows Live how the company is thinking about making Windows Live appeal to third-party developers, not just to Microsoft's own product teams.

Microsoft has been mulling how best to articulate its Windows Live developer story for several months, as noted on the LiveSide.Net Web site. To help simplify its message, Microsoft has consolidated its content and tools for Windows Live developers on a single Microsoft Developer Network site.

"We are opening the Windows Live platform to third parties to create a virtuous ecosystem" for users, developers, partners, advertisers and Microsoft, said Brian Arbogast, corporate vice president for the MSN Communications Platform with the MSN and Personal Services Division.

Arbogast outlined for session attendees the set of programming interfaces that Microsoft now considers the core of its Windows Live developer platform.

At the "core" level, Arbogast said, Microsoft is making available to developers three sets of interfaces: contacts, identity and storage. Microsoft is expecting developers to build on top of these when devising new Windows Live services.

On top of that, Microsoft is offering developers an optional layer of Windows Live "common services," which include Windows Live Search, AdCenter, Presence, Mapping and Mobile interfaces. Developers can follow Microsoft's own lead and embed these interfaces in any or all of the Windows Live services they design.

In addition, Microsoft is offering Windows Live applications and experiences, including Windows Live Mail, Windows Live Messenger, MSN Spaces, Windows Live Marketplaces, Windows Live video and Xbox Live gaming. The company is looking to third-party developers to build similar services, Arbogast said.

Until recently, all of Microsoft's growing family of Windows Live services were considered to be application/end-user offerings. But this is changing, Arbogast said. Now, some Live services, such as Search and Contacts—which Arbogast defined as a "store for relationships, attributes and permissions models"—are considered part of the Live infrastructure.

In Microsoft's world view, developers can build their own "Windows Live experiences," Arbogast said. They also will have the option of developing rich-client applications and/or Web sites which incorporate various Windows Live services and/or programming interfaces directly into them, he said.

Microsoft is still thinking through the business models and licensing models that will be permitted in the Windows Live world, Arbogast said. But the company has decided that a few key principles will prevail.

Users must be in control of their own data at all times, Arbogast said. Windows Live services should be designed to support any platform, browser, language or device, and Windows Live services should make use of simple, standards-adherent HTTP-based application programming interfaces, he added.

The overarching goal will be to make licensing terms more liberal, Arbogast said.

"We'll provide free access without great limits wherever possible," he said. "When it's harder to split the commercial aspect of the API, we will have some limits."

Arbogast said Microsoft will expect developers to choose their own business models when operating outside the Windows Live properties realm, but to refrain from displaying external ads when running inside the Windows Live boundaries.