The licence that one releases their software under is often a topic that’s given less thought then it perhaps deserves. I’ve been releasing my code under the MIT (Expat) licence for as long as I can remember, but I thought it might be prudent to take a closer look at what’s out there in the wild world of software licensing.
Unless otherwise specified, neither this page, nor any of the pages that I’m about to link to, necessarily constitute Proper Legal Advice™. If your project matters enough that the choice of its licence may have a significant effect, please hire an actual lawyer. This means that you should definitely steer clear of Lionel Hutz.
If you want to choose a licence, and you want to make the most reasonable choice you can in the least possible time, use GitHub’s Choose a License. For most scenarios, the choice basically boils down to Expat, Apache 2.0, or the GPL.
If you’re looking for a comprehensive reference on the pros and cons of the licence you’re considering, check out the Free Software Foundation’s Various Licenses and Comments about Them, which is only occasionally didactic about copyleft licensing. You may also find TLDRLegal and OSS Watch’s Licence differentiator useful.
Whether or not two licences are “compatible” is important for when programs of those licences are combined, derived, or linked together, but the details depend on who you ask. For some, compatibility is achieved when works of the given licences may be combined to form a larger software project, while for others, compatibility also requires that one of the constituent works’ licences be able to “dominate”, licensing the entire derived work. Compatibility is generally not a symmetric relation.
The public domain
Placing your work in the public domain is simple, in theory, as doing so should relinquish your ownership and all of your rights over said work. Very few jursdictions, however, provide a practical way to do this, and in some jurisdictions, you may not be allowed to do this at all! For people in those places, your work will effectively continue to be “all rights reserved”, rendering it unusable for them.
Enter CC0. It’s essentially a licence that attempts to waive all rights over a given work in a given jurisdiction, and failing that, places the work under a licence with the most permissive terms allowed in that jurisdiction.
CC0 is usually referred to as a “tool” instead of a “licence”, because where a work may be placed in the public domain, CC0 no longer bears the role of a licence. Unlike most Creative Commons licences, CC0 is suitable for use with any kind of work, including software. The FSF prefers CC0 for releasing works into the public domain.
The novelty licences
Other attempts to solve the issue of placing works in the public domain, include Banlu Kemiyatorn and Sam Hocevar’s Do What the Fuck You Want to Public License, or WTFPL for short. While on the surface, writing a licence to informally relinquish all rights seems like a reasonable solution, the Open Source Initiative has noted that without a clear and explicit grant of rights, the WTFPL is no more universal than any other lone attempt to place a work in the public domain.
By lacking any explicit legal terms, the WTFPL also fails to disclaim authors’ warranties and liabilities for their software, protections that are provided by virtually all serious free and open licences. As such, while “novelty” licences like the WTFPL are certainly amusing, they’re not a wise choice for any significant projects.
The copyleft licences
In an effort to ensure that free and open works remain so, even after they’re distributed or modified to create derivative works, “copyleft” licences like the GNU General Public License were created. Richard Stallman described the concept succinctly in his GNU Manifesto: “no distributor [of the GNU operating system] will be allowed to restrict its further redistribution”.
When distributing software released under a copyleft licence, whether or not any modifications have been made, the source must be included or otherwise made readily available along with any binaries or other forms of the software.
Some examples of copyleft licences include the GNU GPL family, the Mozilla Public License family, as well as Sun Microsystems’ derivatives of the MPL, the Sun Public License and the Common Development and Distribution License.
While version 2.0 of the MPL has been updated to explicitly introduce compatibility with some of the GPL licences, the SPL and the CDDL are based on version 1.0 and 1.1 of the MPL, and this is — at least in part — why they’re incompatible with the GPL.
The permissive licences
Free and open licences which are not copyleft are usually referred to as “permissive” licences. Like all free and open licences, permissive licences adhere to the Free Software Definition, and programs licensed under a permissive licence grant anyone the freedom to:
- Run the program in any manner, and for any purpose;
- Study how the program works, and change it to suit one’s needs;
- Redistribute copies (of the original program) to others; and
- Distribute copies of one’s modified versions to others.
Permissive licences almost always specify a common set of additional terms:
- The authors disclaim all warranties and liabilities for their software;
- Copies and derivative works must credit the original authors; and
- Copies and derivatives must retain the original copyright notices.
There are loads of permissive licences out there, including but not limited to:
- The MIT licences (Expat, X11, and XFree86);
- The BSD licences (“prior”, “original”, “revised”, “simplified”, and FreeBSD);
- The University of Illinois/NCSA licence (UIUC or NCSA for short);
- The ISC licences (with or without the FSF’s clarification); and
- The Apache licences (versions 1.0, 1.1, and 2.0).
They’re all worth mentioning here, because they share a handful of subtle differences.
Advertising and endorsement clauses
Being permissive licences, the MIT, BSD, NCSA, ISC, and Apache families of licences are nearly identical in their spirit and terms.
Chronologically speaking, there are five licences that have been used in the BSD and its derivatives, although only the last three are really relevant for modern usage:
- The “prior” BSD licence (1988), as used by 4.3BSD-Tahoe;
- The “original” BSD licence (1990), with four clauses;
- The “revised” BSD licence (1999), with three clauses;
- The “simplified” BSD licence, with two clauses; and
- The FreeBSD licence, which also has two clauses.
The earliest two versions of the licence are roughly equivalent, and they have clauses that are known as the “advertising clauses”, which require that any advertising for products or services that contain licensed code must include an acknowledgement of its authors.
You might correctly imagine that this requirement can prove very unwieldy, very quickly, and as such, they aren’t approved by the OSI, nor are they compatible with many popular licences like the GPL. The “revised” version removes the advertising clause, making it far less troublesome and incompatible with other free and open licences.
All three of these versions, however, include another clause that requires permission prior to using the names of a work’s authors and contributors to “endorse or promote” derivative works. Contrary to my initial assumption, this doesn’t preclude the “revised” version from compatibility with the GPL, nor from being considered free and open.
The “simplified” BSD licence removes this “endorsement” clause too, leaving behind a fairly concise permissive licence, and the FreeBSD licence is based on this version, with the addition of a paragraph stating that the “views and conclusions” of the individual authors don’t represent the FreeBSD Project as a whole.
As for the MIT family of licences:
- The Expat licence is similar to the “simplified” BSD licence;
- The X11 licence is similar to the “revised” BSD licence; and
- The XFree86 licence is similar to the “original” BSD licence.
In other words, the X11 licence is like the Expat licence plus an endorsement clause, and the XFree86 licence is like the X11 licence plus an advertising clause.
The NCSA licence, put simply, is based on the “revised” BSD licence, with some minor improvements in wording drawn from the X11 licence to improve precision.
Version 1.0 of the Apache licence is similar to the “original” BSD licence by virtue of having an advertising clause, while version 1.1 removes the clause, making it vaguely like the “revised” BSD licence. These versions of the Apache licence weren’t really designed for general use by other projects however, and their strict and specific prohibitions on the use of Apache-related names, along with their incompatibility with the GPL because of these, make them rather unattractive options.
Finally and most elegantly, the ISC licences are “functionally equivalent” to the Expat and “simplified” BSD licences, but thanks to the Berne convention, they’re over 30% less wordy than Expat, and over 40% less wordy than the BSD two-clause!
Patent grants and retaliation clauses
Most of the permissive licences we’ve discussed so far were written before software patents were widespread. With these licences, anyone who uses a program that’s covered by one or more patents may risk being the target of litigation from patent holders.
There are two kinds of clauses that a software licence can use to help mitigate this risk: grant clauses grant users a licence to use any necessary patents, and retaliation clauses revoke these patent licences from anyone who initiates litigation.
Enter version 2.0 of the Apache licence. This licence contains both a grant clause and a retaliation clause, making it widely recommended as the most robust licence with respect to its protections against patent litigation. The retaliation clause alone subsequently made its way into the GPL 3.0 and the MPL 2.0.
The Creative Commons licences
Mainly geared towards artistic and other creative works, the Creative Commons family of licences give authors a variety of choices about how their works may be used. Regardless of the choices that a given author makes, anyone may — at the very least — distribute any work that’s in the Creative Commons worldwide, but:
- The work must not be modified, regardless of its distribution;
- Attribution for the authors of the work must be preserved; and
- The distribution must not take place for commercial purposes.
From here, authors may tweak the terms of the licence by:
- Allowing the commercial distribution of their work; and/or
- Allowing derivative works:
- Provided that they use the same licence; or
- Regardless of the resultant licence.
It’s not a good idea to use any of the Creative Commons licences, except for CC0, for software projects, because they lack specific terms that are desirable for software licensing, and they’re incompatible with most free and open software licences, among other reasons.
When readable source isn’t open source
That one is allowed to read the source code of a work alone doesn’t imply that the FSF will consider its licence “free”, nor will the OSI necessarily call its licence “open”. In the case of the OSI, the availability and legibility of source code merely forms a part of one of the ten criteria that the Open Source Definition comprises.
One example of such a licence is the TrueCrypt licence, which not only has excessive restrictions on how licensed works may be derived from or distributed, but also forbids anyone who doesn’t understand the licence from using a licensed work, which is a clear violation of freedom 0 of the Free Software Definition.
Another licence that fits the bill here is the JSON licence, which Douglas Crockford uses for many of his popular programs such as JSLint and JSMin. It’s identical to the Expat licence, with the exception of an additional sentence: “The Software shall be used for Good, not Evil.”
While many people may agree that this additional “restriction” is clearly tongue-in-cheek, as it’d be ineffective at preventing any actual evildoers from using a licensed work to do their evils, it makes the JSON licence a nightmare to use for any meaningful project.
The subjective nature of evil could potentially put users of a licensed work at risk of being a target of litigation without their knowledge, and this restriction is also a clear violation of freedom 0, making it incompatible with essentially all free and open licences.
Just as the Creative Commons, GPL, MPL, and Apache licences must be disambiguated with a version and possibly some licensing settings, the “MIT licence” and “BSD licence” need to be qualified with which one of the three or five variants they encompass is being discussed respectively.
Referring to the NCSA licence isn’t at all ambiguous, and the only ambiguity when discussing the ISC licence is the clarifying replacement of “and distribute” with “and/or distribute” to allow the licence to be approved by the FSF, because some projects like OpenBSD continue to use the old wording.
Vanity and aesthetics
It’s also interesting to see that these free and open licences are usually named after:
- A software project: GNU GPL, MPL, Apache, BSD, Expat, X11, XFree86;
- A university: MIT, UIUC/NCSA;
- Another organisation: Creative Commons, ISC; or
- Its contents: WTFPL.
While you could chalk it up to me being a little bit weird, I have a slight but irrational aversion to preferring licences that fall into the first two categories.
Relicensing my work
At the exceedingly rare risk of these being foreshadowing words, I don’t see myself being the target of patent litigation any time soon, and I don’t feel like a copyleft licence provides a meaningful benefit for my code over a permissive one.
I think I’ll relicense my projects under the ISC licence over the next week or two — not that it matters, as virtually nobody actually uses the code I’ve written, but because it’s cute as fuck and it shares its semantics with the Expat licence that I’m currently using.