Namespace semantic differences

I see most of the types in .NET framework are spread across 3 different namespaces (may be more), one start with Microsoft, other with System and the third with Windows.

For example there is Windows.System.Threading.ThreadPool and System.Threading.ThreadPool.

Is there a clear cut semantic difference on this design?

Jon Skeet
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Microsoft.* namespaces are typically .NET namespaces for features which are specific to Windows, e.g. registry access

System.* namespaces are "normal" .NET system namespaces

Windows.* namespaces are typically part of Windows Runtime aka WinRT (for Windows 8 store apps, Windows Phone 8)

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