Windows 9.1

Windows 9.1 was a cancelled project due to the release of Windows 8.2. It's planned to be a upgrade for Windows 9 (like Windows 8.1 is upgrade of Windows 8. There's some leaked builds.

Build 9642
This build leaked on May 27, 2014 in internet.

Build 9651
Leaked on June 13, 2014 by a hacker. In this build, modern UI apps can be open in a normal window (like Windows 10) but can switch between full screen and windowed.

Build 9701
This is the last build before it was cancelled, leaked in late June 2014.