â 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 . Other key components of a software system may use other licenses ; many libraries use the GNU Lesser General Public License ( LGPL ) , a more permissive variant of the GPL , and the X.org implementation of the X Window System uses the MIT License .