Week 6      Copyleft

Copyleft grants the right to freely distribute and modify intellectual property with the requirement to keep the same rights in derivative works created on the basis of that property. Copyleft also helps programmers who want to make improvements to free software to get permission to do so. Copyleft is a way of applying copyright to a program. It does not mean a complete waiver of copyright; otherwise, it would make copyleft impossible.

The term “weak copyleft” refers to licenses in which not all derivative works inherit a copyleft license. Whether a derivative work is inherited or not often depends on how it was obtained. Weak copyleft licenses are often used to create software libraries. Only changes to the software itself licensed with a weak copyleft are subject to the terms of such Copyleft license.This allows programs of any license to be compiled and linked with copyleft libraries such as glibc, and then redistributed without relicensing. Free software licenses using "weak" Copyleft include the GNU Lesser General Public License and the Mozilla Public License.

The best known strong copyleft free software license is the GNU Copyleft General Public License .The goal of the GNU Project was to give all users the freedom to redistribute and modify GNU software. The design science license is also strong copyleft license , as it can be applied to any work, not just software or documentation, but also to literature, art, music, photography, and video as well. The scientific design license was created by Michael Stutz after he showed interest in applying GNU copyleft to non-software works, which later became known as open source copyleft content .

Permissive software licenses are those that grant users of the software the same freedoms as copyleft licenses, but do not require modified versions of that software to include those freedoms as well. They have minimal restrictions on using, modifying, and distributing the software, and therefore are not copyleft licenses. Examples of this type of license include the X11 license, the Apache license, the MIT license, and the BSD license.

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