Windows Phone 7.1 is the version number of the OS and developer SDK for Windows Phone "Codename Mango". Phones running this operating system are marketed as being "Windows Phone 7.5".
The codename "Mango" update to Windows Phone 7. Made available to developers in June 2011 and scheduled for public availability later in 2011. It contains over 500 new features and 1500+ new APIs.
This tag can be considered a synonym of [windows-phone-7.5] as they both refer to the same thing.
Use of the generic tag [windows-phone-7] is recommended as more people are likely to see such questions. This, hopefully, increases your likelihood of a fast and accurate answer.
More on the confusion between 7.1 & 7.5 can be found at:
- Windows Phone “Mango” – Released to Manufacturing
- Microsoft gives Windows Phone developers a refreshed (non-RTM) Mango build
- https://meta.stackexchange.com/questions/100126/windows-phone-codename-mango-and-version-numbers
- https://meta.stackexchange.com/questions/100602/change-windows-phone-7-to-windows-phone
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See windows-phone for more information.