Copyleft

Copyleft is a licensing scheme that uses copyright law to ensure that modified versions of a work are also free. It's a method of granting permission to use, modify, and distribute a work, but requiring that all modified and extended versions of that work also be under the same or similar copyleft license.

The main goal of copyleft is to protect the freedom of a work and its derivatives from becoming proprietary. This is achieved by stipulating that if someone distributes a modified version of the copylefted work, they must also distribute it under the same copyleft license, thus ensuring the downstream freedom of the work.

Copyleft licenses typically allow anyone to use, study, share (copy), and modify the work. The critical difference between copyleft and permissive licenses is that copyleft mandates that any derived works must also be licensed under the same copyleft license, preserving the original freedoms. Permissive licenses, on the other hand, allow derived works to be licensed under proprietary terms.

Copyleft is often associated with software, but it can be applied to other types of creative works such as documentation, art, and music. The GNU General Public License (GPL) is a well-known example of a copyleft license commonly used for software.

There are different types of copyleft licenses. Strong copyleft requires that all derivative works, even those that are only loosely linked to the original work, must be licensed under the same copyleft license. Weak copyleft, on the other hand, typically only applies to the parts of the derivative work that are directly derived from the original work; other parts of the derivative work can be licensed under different terms.

The concept of copyleft aims to create a commons of freely available works that can be built upon and improved by anyone, ensuring that the freedom to use, modify, and share these works is preserved for future generations. It leverages the existing legal framework of copyright to achieve this goal.