, modifications, and redistribution of the software. In particular, permissive licenses do not attempt to guarantee that future versions of the software will remain open and free. In contrast to the permissive license, a copyleft license grants a user full rights to modify and redistribute the software, with the limitation that any derivative work based on copyleft software must also be copyleft - that is open and free. Here we elaborate on just a few of the most common open source licenses: Perhaps the most important (but not necessarily the most widespread as of now) open source license is the GNU General Public License (GPL) family. For example, GPLv2 is the license used by the Linux kernel. It is considered a "copyleft" open source license that 6.3 Open Source guarantees end users the freedom to run, study, share, and modify the software. How- ever, it imposes a number of important restrictions; for example, that derivative work must be open source and distributed under the same license terms. It was initially writ- ten by Richard Stallman for his GNU project in 1989. The second, conceptually similar, version was released in 1991. However, after GPLv2 was released, several members of the open source community realized that GPLv2 can be exploited by allowing the inclusion of GPL-licensed software in custom hardware that prevents users from running modified versions of its software - the practice sometimes referred to as "tivoization" (Tivoiza- tion). GPLv3 was released to address that issue; however, it never became as popular as GPLv2, in particular because Linus Torvalds refused to adopt it for the Linux kernel. Once the most popular open source license, GPL has become much less widespread in recent years, perhaps in part due to the fact that most open source code is now written by people employed by large corporations, as opposed to volunteers. One of the earliest permissive licenses is the Massachusetts Institute of Technology (MIT) License, introduced circa 1990. Unfortunately, the term "MIT license" is s