To increase revenues, a general trend began to no longer distribute source code (easily readable by programmers), and only distribute the executable machine code that was compiled from the source code. One person especially distressed by this new practice was Richard Stallman. He was concerned that he could no longer study or further modify programs initially written by others. Stallman viewed this practice as ethically wrong. In response, he founded the GNU Project in 1983 so that people could use computers using only free software. He established a non-profit organization, the Free Software Foundation, in 1985, to more formally organize the project. He invented copyleft, a legal mechanism to preserve the "free" status of a work subject to copyright, and implemented this in the GNU General Public License. Copyleft licenses allow authors to grant a number of rights to users (including rights to use a work without further charges, and rights to obtain, study and modify the program's complete corresponding source code) but requires derivatives to remain under the same license or one without any additional restrictions. Since derivatives include combinations with other original programs, downstream authors are prevented from turning the initial work into proprietary software, and invited to contribute to the copyleft commons. Later, variations of such licenses were developed by others.