Copyleft

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 title: What is copyleft?
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+
+**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&nbsp;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 >}}