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diff --git a/content/learn/copyleft.md b/content/learn/copyleft.md index faa3d92..b3a85ef 100644 --- a/content/learn/copyleft.md +++ b/content/learn/copyleft.md @@ -2,3 +2,109 @@ title: What is copyleft? weight: -8 --- + +**Copyleft** is a licensing tool unique to free software. It is designed to +encourage the proliferation of free software and protect free software from +being incorporated into non-free works. This works by giving you not only the +*right* to share your improvements, but the *obligation* to share your +improvements, under some conditions. It is very important to understand these +obligations when re-using copyleft software in your own work. + +{{< tip >}} +**Terminology note**: +Conflating "copyleft" and "free software" is a common misconception: +non-copyleft software can be Free Software, and Open Source software can be +copyleft. However, those who identify more closely with the Free Software +movement tend to favor copyleft licenses more than those who identify with the +Open Source movement. +{{< /tip >}} + +## The copyleft spectrum + +Different free software licenses exist along a spectrum from **permissive** to +**copyleft**, based on the degree to which they emphasize copyleft in their +license terms. A permissive license will tend to allow generous reuse with +relatively few and non-onerous obligations, such as simple attribution +requirements. In contrast, copyleft licenses impose the obligation to share your +changes and derived works under the same license terms. + +<img src="/images/licensing-spectrum.svg" alt="various projects and licenses organized along a spectrum" /> +<small> + Various software licenses and projects which use them, organized on the + copyleft spectrum. Based on graphic by David A Wheeler, CC-BY-SA 3.0. +</small> + +## Why choose a copyleft license? + +It is common for permissively-licensed free software to be incorporated into +non-free works. This work is often leveraged for profit, denying the four +freedoms to users who receive the non-free work, and making profitable use of +the software without giving anything back to the free software community. + +Copyleft licenses answer some of these problems: + +1. Copyleft promotes the proliferation of free software and the four freedoms by + ensuring that work built on top of free software grows and benefits the free + software ecosystem. +2. Copyleft ensures that those who improve free software share their changes + with the community, so that all users can benefit from their improvements. + +Copyleft software can be sold, like all other free software, but requiring that +commercial improvements remain free ensures the four freedoms are upheld by all +participants. Furthermore, it is difficult to change the license of copyleft +software if the copyright is held [in aggregate][0], which serves as a +strong promise for the future of the software as free software. + +[0]: /learn/participate/copyright-ownership/ + +## Weak and strong copyleft + +Copyleft licenses differ in how strongly their copyleft clauses affect re-use of +the software. For example, the weak copyleft [Mozilla Public License][MPL] is +*file-based*, such that the copyleft clause covers individual source code files, +and not the project as a whole: you can drop one of these files into any project +without having to relicense the larger project, so long as you re-publish any +changes to those specific files. + +[MPL]: https://www.mozilla.org/en-US/MPL/2.0/ + +A somewhat stronger example is the [GNU Lesser General Public License][LGPL], +which deals specifically with software libraries. These libraries are compiled +into an aggregate software artifact, such as a shared object or static archive, +and the copyleft terms applies to this entire artifact. However, when this is +linked with a third-party program, the copyleft clause is not invoked. Stronger +still is the [GNU General Public License][GPL], which treats the completed +program as the software artifact to which the copyleft clause applies. + +[LGPL]: https://www.gnu.org/licenses/lgpl-3.0.en.html +[GPL]: https://www.gnu.org/licenses/gpl-3.0.html + +On the far end of the copyleft spectrum are licenses like the [GNU Affero +General Public License][AGPL], which extends the <abbr title="GNU General Public +License">GPL</abbr> to apply to software used over a network, such as databases, +and considers users of that software "recipients" of the software, who are thus +entitled to receive the source code. + +[AGPL]: https://www.gnu.org/licenses/agpl-3.0.html + +## How to re-use copyleft works + +The simplest way to re-use copyleft works is to apply its license to your own +work and distribute it accordingly. + +If you do not want to do this, you can only use a copyleft work under the +conditions permitted by its license, and will likely be limited to the use of +weak copyleft works. For example, if your software depends on a library which +uses the <abbr title="GNU Lesser General Public License">LGPL</abbr>, you may +use any license for your own work but need to share changes you make to the +library itself. If the software uses the GPL or AGPL, you will be more +constrained in your approach. Read the license terms carefully and consult a +lawyer if you are unsure how to proceed. + +{{< tip >}} +The [Software Freedom Conservancy][sfc] is an organization which, among other +activities, pursues legal remedies for copyleft enforcement. To learn more about +copyleft enforcement for your own projects, consult their resources. + +[sfc]: https://sfconservancy.org/ +{{< /tip >}} |