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Windows RTAn earlier version of the Windows 8 operating system for ARM-based tablets. Pre-installed on tablets, RT apps were available only from Microsoft's online Store. Introduced with Windows 8 in 2012, by 2015 there was no upgrade path for RT users (see "Why Two Tablet Versions" below).
Windows RT included desktop versions of Microsoft Office 2013, Internet Explorer and File Explorer, all of which were the only desktop apps with overlapping, resizable windows that ran in RT tablets. All other apps were full-screen tablet style. See Windows 8.
Windows RT Did Not Run Legacy Apps
Applications that ran on Windows XP, Vista and Windows 7 did not run on Windows RT tablets, something novice users surprisingly discovered if they were not forewarned in the store. Only Windows 8 tablets ran "legacy" Windows apps.
Why Two Tablet Versions?
Throughout the Windows 8 development time frame, the ARM CPU chips that power most mobile devices were more efficient and less expensive than Intel's x86 CPUs. In order for Microsoft to compete in the tablet market, users were offered two Windows 8 tablets: lower-priced Windows RT/ARM tablets that run only RT apps and higher-priced Windows 8/Intel tablets that ran all Windows desktop apps. RT tablets caused confusion in the marketplace, and their inability to run regular Windows programs doomed their future. Microsoft subsequently resurrected Windows on ARM CPUs in 2016 (see Windows 10 on ARM).
Windows RT vs. WinRT
Windows RT was the tablet operating system. WinRT was the runtime (RT) engine used with Windows RT and Windows 8 to deliver tablet-style (Metro) apps. See WinRT, Surface tablet, ARM and Office 2013.