The Linux kernel and most GNU software are licensed under the GNU General Public License (GPL). The GPL requires that anyone who distributes the Linux kernel must make the source code (and any modifications) available to the recipient under the same terms. In 1997, Linus Torvalds said, "Making Linux GPL'd was definitely the best thing I ever did". Other key components of a Linux system may use other licenses; many libraries use the GNU Lesser General Public License (LGPL), a type of the GPL that is less restricted, and the X Window System uses the MIT License. "Linux" is a trademark of Linus Torvalds.