Microsoft and Foursquare have signed a four-year partnership with far-reaching implications for anyone making or using apps that taps into users’ locations, Foursquare CEO Dennis Crowley told ReadWrite in an interview Monday evening. The companies plan to announce the deal formally Tuesday afternoon.

Already, the Wall Street Journal and the New York Times have reported on the financial aspects of the deal: Microsoft is making a $15 million investment in Foursquare, an extension of a $35 million financing round Foursquare announced late last year, valuing the company at approximately $650 million. Microsoft will also pay Foursquare to license its data, which is sourced from users’ check-ins, or announcements of their locations to designated friends.

Dollars will help Foursquare, whose resources were stretched until it borrowed $41 million from investors last spring. But far more interesting is the potential for Foursquare to weave its location services into Microsoft's platforms, including Windows, Windows Phone, and the Bing search engine.

A Location Bake-Off

Foursquare CEO Dennis Crowley said details of the partnership, freshly struck, are still being drawn. "Imagine a bunch of present-tense and future-tense Microsoft products that have a little Foursquare touch to them,” he said.

It’s easy to imagine Bing search results for places, for example, having information about businesses that comes from Foursquare.

A far more interesting prospect is what Microsoft and Foursquare can do for a large group of joint customers—developers. Some 50,000 developers use Foursquare's application programming interface to add location features to their apps, according to Crowley.

But most simply use the API to “geotag,” or add a place name, to images, videos, or other forms of content, Crowley said. Instagram, the Facebook-owned photo-sharing app, is the most famous example.

"Foursquare's great for that, but that's like a tenth of what Foursquare is doing,” he said. Instead, developers could query Foursquare’s database and discover a user’s relationship to a place—how frequently they check in, for example, and at what times of day.

That kind of data, unique to Foursquare, could let a calendar app recommend a restaurant for a lunch appointment, or allow a travel-booking app to add a list of recommended sights to an itinerary. In short, it could power the kind of experiences iOS users get through Apple’s Siri feature—but now on Microsoft's platforms.

Crowley calls those kinds of features "contextual awareness”—it’s also known as “anticipatory computing”—and building that kind of smarts into Microsoft's platforms is one of the ambitions of the deal.

The deal is not exclusive, so Foursquare is keeping open the possibility of striking similar deals with Apple and Google for their mobile platforms.

“We think about things like, ‘If Foursquare is baked into the [operating system] at some level, what would that look like?'" Crowley said, describing internal conversations at Foursquare.