He pioneered the concept of 'copyleft', which uses the principles of copyright law to preserve the right to use, modify and distribute free software, and is the main author of free software licenses which describe those terms, most notably the GNU General Public License (GPL), the most widely used free software license.
Stallman ngameloporin konsep copyleft, sané nganggén parikrama-parikrama hukum hak ripta antuk ngelindungin hak antuk nganggén, nguahin, miwah nyebarang piranti lunak bebas, miwah penyurat utama lisensi perangkat lunak bebas sané nganikayang uger-uger punika, kautama GNU General Public License (GPL), lisensi piranti lunak bebas pinih akéh keanggén.[7]