Introduction

Providing legal protection for Free Software is possible using copyright law. It turns out that granting freedom to software users is a bit more complex than denying them all freedom. The license under which a Free Software program is distributed must therefore be designed with great care to ensure that the author's wishes are respected.

In the following you will find a description of the Copyleft concept that can be viewed as an introduction to the reasoning that someone has to make to provide protection for Free Software. The path to actually write a license based on this reasoning is hard and long. Copyleft is not the only way to think about Free Software licenses. The X11 license or the Berkeley Database License were designed using a different logic and are well fit for Free Software.

Although some individuals and companies try to undertake this enormous task, most of them realize that using an existing Free Software license is much easier and much safer. A list of known Free Software licenses is provided below so that you can find the one that best suits your purpose. When doing so you need to understand the benefits of each license and be careful about compatibility issues.

Some Free Software licenses

The (compat) sign is added for each license which is compatible with the GNU GPL license. For more information see the section relating to license compatibility below.

• The GNU General Public License, or GNU GPL for short.
• The GNU Lesser General Public License, or GNU LGPL for short. (compat)
• The license of Guile. (compat)
• The license of the run-time units of the GNU Ada compiler. (compat)
• The X11 license. (compat)
• The Cryptix General License. (compat)
• The modified BSD license. (compat)
• The license of ZLib. (compat)
• The license of the iMatix Standard Function Library. (compat)
• The W3C Software Notice and License. (compat)
• The Berkeley Database License (aka the Sleepycat Software Product License). (compat)
• The License of Python 1.6a2 and earlier versions. (compat)
• The License of Python 2.0.1, 2.1.1, and newer versions. (compat)
• The license of Perl. (compat)
• The Clarified Artistic License (used by NcFTP). (compat)
• The license of Netscape Javascript. (compat)
• The Arphic Public License.
• The original BSD license.
• The Apache License, Version 1.0.
• The Apache License, Version 1.1.
• The Zope Public License.
• The license of xinetd
• The License of Python 1.6b1 and later versions, through 2.0 and 2.1.
• The OpenLDAP License, Version 2.3.
• IBM Public License, Version 1.0
• The Phorum License, Version 1.2
• The LaTeX Project Public License.
• The Mozilla Public License (MPL).
• The Netizen Open Source License (NOSL), Version 1.0.
• The Interbase Public License, Version 1.0.
• The Sun Public License.
• The Nokia Open Source License.
• The Netscape Public License (NPL)
• The Jabber Open Source License, Version 1.0
• The Sun Industry Standards Source License 1.0
• The Qt Public License (QPL).
• The FreeType license
• The Open Compatibility License
• The PHP License, Version 2.02.

For the impatient

If you need to take an immediate decision and can't spare time to educate yourself on the licensing subject, it is recommended to follow the advice listed below. This is overly-conservative advice that you can apply if you want to make sure that your freedom won't be jeopardized. However, it is very important to learn more about the legal protection of Free Software shortly afterwards. Reading this document and following links will introduce the concepts and enable you to make up your mind. It would be a mistake to consider this advice as a definitive way of dealing with legal issues.

• Only deal with software covered by the GNU GPL or GNU LGPL and documentation covered by the GNU FDL
• Don't attempt to write a Free Software license
• Don't attempt to find out if a license is Free Software

What Is Copyleft?

Copyleft is a general method for making a program Free Software and requiring all modified and extended versions of the program to be Free Software as well.

The simplest way to make a program free is to put it in the public domain, uncopyrighted. This allows people to share the program and their improvements, if they are so minded. But it also allows uncooperative people to convert the program into proprietary software. They can make changes, many or few, and distribute the result as a proprietary product. People who receive the program in that modified form do not have the freedom that the original author gave them; the middleman has stripped it away.

Let's say that your aim is to give all users the freedom to redistribute and change the software you wrote. If middlemen could strip off the freedom, you might have many users, but those users would not have freedom. So instead of putting your software in the public domain, you can ``copyleft'' it. Copyleft says that anyone who redistributes the software, with or without changes, must pass along the freedom to further copy and change it. Copyleft guarantees that every user has freedom.

Copyleft also provides an incentive for other programmers to add to free software. Important free programs such as the GNU C++ compiler exist only because of this.

Copyleft also helps programmers who want to contribute improvements to Free Software to get permission to do that. These programmers often work for companies or universities that would do almost anything to get more money. A programmer may want to contribute their changes to the community, but their employer may want to turn the changes into a proprietary software product.

When it is explained to the employer that it is illegal to distribute the improved version except as Free Software, the employer usually decides to release it as Free Software rather than throw it away.

To copyleft a program, you first state that it is copyrighted; then you add distribution terms, which are a legal instrument that gives everyone the rights to use, modify, and redistribute the program's code or any program derived from it but only if the distribution terms are unchanged. Thus, the code and the freedoms become legally inseparable.

Proprietary software developers use copyright to take away the users' freedom; you can use copyright to guarantee their freedom. That's why the name is reversed, changing ``copyright'' into ``copyleft.''

Copyleft is a general concept; there are many ways to specify the details. In the GNU Project for instance, the specific distribution terms are contained in the GNU General Public License, the GNU Lesser General Public License and the GNU Free Documentation License.

The appropriate license is included in many manuals and in each GNU source code distribution.

The GNU GPL is designed so that you can easily apply it to your own program if you are the copyright holder. You don't have to modify the GNU GPL to do this, just add notices to your program which refer properly to the GNU GPL. Please note that you must use the entire text of the GPL, if you use it. It is an integral whole, and partial copies are not permitted. (Likewise for the LGPL.)

Using the same distribution terms for many different programs makes it easy to copy code between various different programs. Since they all have the same distribution terms, there is no need to think about whether the terms are compatible. The Lesser GPL includes a provision that lets you alter the distribution terms to the ordinary GPL, so that you can copy code into another program covered by the GPL.

Compatibility between licenses

A compatibility study is ongoing for the most wide spread license, the GNU GPL. It lists licenses that are compatible with it and the licenses that are incompatible. Although this on-going study is a good reference, every software author has to think about the implications of using a specific license. Choosing a license that has compatibility problems may lead to practical troubles and prevent the use of the software in conjunction with others.

Rationale behind the choice of a license

The best way to find which Free Software license is fit for the software you wrote is to talk to people. Send mail to non-profit Free Software organizations explaining your situation, weigh their answers and make up your mind. Always try to speak with at least one person with actual experience, someone who had to face the same questions in the past.

Experience shows that choosing a Free Software license is only partly a matter of evaluating advantages and drawbacks. Most of the discussions surrounding this choice lead to a better understanding of the Free Software movement. Once the level of understanding reaches a given level, the choice of the license is often straight forward.

Beware of translations

The strongest incentive to use the translation of a license is to better comply to national laws. This is rarely needed, unless you live in a country that did not sign the Berne convention or made significant exceptions to it.

In some cases government agencies are forced to use licenses written in the official language of the country. There is no easy solution to this problem at present.

References

The Various Licenses and Comments about Them page complements the list of licenses presented in this page.

The translation effort of the GPL to French.

[Authors: FSF Europe]