A version of the Windows 8 operating system for ARM-based tablets. Pre-installed on tablets, RT apps are available only from Microsoft's online Windows Store. Introduced with Windows 8 in 2012, as of 2015, there was no upgrade path for Windows 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 are the only desktop apps with overlapping, resizable windows that run in Windows RT tablets. All other apps from the Windows Store are full-screen tablet style. See Windows 8. Windows RT Does Not Run Legacy Apps Applications that run on Windows XP, Vista and Windows 7 will not run on Windows RT tablets, something novice users may surprisingly discover if they were not forewarned in the store. Only Windows 8 tablets run "legacy" Windows apps. Why Two Tablet Versions? Throughout Windows 8 development, the ARM CPU chips that power most mobile devices were more efficient and less expensive than x86 Intel 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 run all Windows desktop apps. Microsoft subsequently resurrected Windows on ARM CPUs in 2016 (see Windows 10 ARM). Windows RT Vs. WinRT Windows RT is the tablet operating system. WinRT is the runtime (RT) engine used with Windows RT and Windows 8 to deliver tablet-style (Metro) apps. See WinRT, Surface tablet, Windows Store, ARM chips and Office 2013.