The MIT License is a permissive free software license originating at the Massachusetts Institute of Technology (MIT) in the late 1980s. As a permissive license, it puts only very limited restriction on reuse and has, therefore, high license compatibility. The MIT license is compatible with many copyleft licenses, such as the GNU General Public License (GPL); MIT licensed software can be re-licensed as GPL software, and integrated with other GPL software, but not the other way around. The MIT license also permits reuse within proprietary software, provided that either all copies of the licensed software include a copy of the MIT License terms and the copyright notice, or the software is re-licensed to remove this requirement. MIT-licensed software can also be re-licensed as proprietary software, which distinguishes it from copyleft software licenses. As of 2020, MIT was the most popular software license found in one analysis, continuing from reports in 2015 that MIT was the most popular software license on GitHub, ahead of any GPL variant and other free and open-source software (FOSS) licenses. Notable projects that use the MIT License include the X Window System, Ruby on Rails, Node.js, Lua and jQuery. Notable companies using the MIT license include Microsoft (.NET Core), Google (Angular) and Facebook (React).