Free software licenses

The four freedoms are generally guaranteed through the use of a free software license. There are many different kinds of licenses with many different trade-offs to suit each software project’s unique situation.

How a free software license works

A free software license grants the necessary rights, perhaps subject to some caveats (e.g. attribution), to establish the four freedoms for recipients of the software. Any software license can be a free software license if it upholds the four freedoms, but in practice most projects pick one of the many popular licenses established for general use. Information about these general-purpose software licenses and how to choose between them for your own projects are addressed in choosing a license.

Often you will encounter a free software license in the “LICENSE” or “COPYING” file present in the software source code. Other projects, particularly those which combine software from many sources, have more complex ways of explaining their licensing situation. A common approach to managing this is the REUSE specification.

If you want to know more about how free software licenses work in detail, read on. Otherwise:

Common traits of free software licenses

To understand your obligations under any particular license, you will have to read it (and perhaps consult a lawyer, especially if you represent a business). However, most free software licenses have some traits in common with others, and you can get a simple understanding of them by learning about a few essential traits. Here are some common features of free software licenses:

Attribution

Attribution clauses require you to attribute the authors when distributing or reusing software based on a license with such a clause. This generally involves reproducing the license in full, or sometimes a simple copyright notice, when you distribute the software, modifications to it, or new software which incorporates some or all of the original software.

Here’s an example from the MIT license:

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

Disclaimer of warranty

Free software is often provided as a gift. In exchange for this gift, you often are asked to agree to accept the software as-is, without any particular expectations of support or warranty from the publisher. This disclaimer of warranty is used to disclaim liability for free software, so the recipient is responsible for what they do with it.

Here’s an example from the MIT license:

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Copyleft

Some licenses don’t just permit you to share your improvements, but require that, if you share the software or software derived from or incorporating parts of the original, you can only do so by using the same license for your improvements. This is a copyleft license: a tool to protect free software from being incorporated into non-free works.

Here’s an example from the Mozilla Public License 2.0:

All distribution of Covered Software in Source Code Form, including any Modifications that You create or to which You contribute, must be under the terms of this License. You must inform recipients that the Source Code Form of the Covered Software is governed by the terms of this License, and how they can obtain a copy of this License. You may not attempt to alter or restrict the recipients’ rights in the Source Code Form.

License compatibility & sublicensing

The ability to combine many works together is an essential trait of the free software ecosystem, but the use of many different copyright licenses can make this work more difficult. This is where sublicensing and license compatibility comes in: many free software licenses make provisions wherein they can be extended by the terms of additional licenses. This allows you to combine software with two or more compatible licenses to produce new software subject to the license terms of both.

Not all licenses have terms which are compatible with one another; in particular copyleft licenses tend to be less compatible with others. Software with incompatible licenses cannot be combined into one work.

Use of trademarks and patents

Software licenses generally deal with copyright-related rights, but commercial software publishers often hold other kinds of intellectual property, such as trademarks and patents. Some free software licenses incorporate clauses which address the relationship between the software’s copyright grant and other intellectual property, for example agreeing that use of the software does not infringe on the copyright holder’s patents, or forbidding the use of the copyright holder’s trademarks.

Here’s an example from the Apache 2.0 license:

3. Grant of Patent License. Subject to the terms and conditions of this License, each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable (except as stated in this section) patent license to make, have made, use, offer to sell, sell, import, and otherwise transfer the Work, where such license applies only to those patent claims licensable by such Contributor that are necessarily infringed by their Contribution(s) alone or by combination of their Contribution(s) with the Work to which such Contribution(s) was submitted. If You institute patent litigation against any entity (including a cross-claim or counterclaim in a lawsuit) alleging that the Work or a Contribution incorporated within the Work constitutes direct or contributory patent infringement, then any patent licenses granted to You under this License for that Work shall terminate as of the date such litigation is filed.