Re-using free software

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 title: Re-using free software
 weight: 90
 ---
+
+One of the great advantages of free software is its potential for re-use. You
+can incorporate code from other free software projects into new projects, saving
+time and allowing you to build on the shoulders of giants. Of course, you have
+to respect the original project's work, and that means complying with their free
+software license terms.
+
+{{< tip "warning" >}}
+Always read the license carefully when incorporating someone else's work into
+your own software.
+{{< /tip >}}
+
+## Incorporating permissive software into new works
+
+The main appeal of permissive software licenses is the ability to incorporate it
+into anything with relatively little obligation towards the copyright holders.
+Most permissive licenses require only that you include the license text, or even
+just a copyright statement, in your product. For free software projects which
+incorporate permissively licensed code into their work, meeting these
+obligations is is usually as straightforward as including an additional license
+with your source code.
+
+When incorporating permissively licensed free software into non-free works, you
+must distribute the free software license and/or copyright attribution with your
+software. Many commercial users of permissively licensed free software include a
+menu somewhere in their product which enumerates the applicable software
+licenses, or include a printed version with the product. You will need to come
+up with a similar approach.
+
+## Incorporating copyleft software into new works
+
+{{< tip "warning" >}}
+Copyleft software **cannot** be incorporated into non-free software.
+{{< /tip >}}
+
+Most free software can be incorporated into copyleft software, and vice versa,
+if the licenses are **compatible**. As a general rule, most popular permissive
+licenses -- but not all -- are compatible with most popular copyleft licenses.
+*Some* copyleft licenses are compatible with other copyleft licenses (for
+instance, the Mozilla Public License 2.0 is compatible with the GNU family of
+licenses), but many are not. Any two projects which use the *same* copyleft
+license are compatible with each other and may share code freely.
+
+{{< tip >}}
+GNU maintains a list of licenses which are compatible and incompatible with the
+GPL family of copyleft licenses [here][0].
+
+[0]: https://www.gnu.org/licenses/license-list.html
+{{< /tip >}}
+
+Incorporating permissive code into a copyleft project is straightforward if the
+licenses are compatible: see the previous section.
+
+{{< tip "warning" >}}
+The inverse, incorporating copyleft software into a permissively licensed
+software project, is less straightforward. In this case, the combined work
+becomes subject to the terms of the copyleft license. Managing a mix of
+permissive and copyleft licenses in a single work is possible, but has important
+and meaningful implications for your project. This is strongly advised against
+for non-experts: don't mix copyleft code into a permissive project unless you
+are prepared for the project to [switch to a copyleft
+license](/learn/participate/copyright-ownership/#changing-a-projects-license).
+{{< /tip >}}
+
+## Managing many licenses and copyrights in one project
+
+Larger and more complex projects may incorporate software from many different
+sources with many different licenses and copyright holders. If this describes
+your project, we recommend that you consider applying the
+[REUSE](https://reuse.software/) specification to your work.