Software Pluralism

Software Pluralism

License Type Overview


The information in this article is intended to assist anyone seeking a general, comparative understanding of the most popular open source licenses currently in use, and the implications of their use.


There are many different licenses for software that is collaboratively developed. Categorizing dozens of different licenses now in use is a daunting task. The Open Source Initiative has made an effort to categorize some licenses as “open” and others as not. See

For purposes of this discussion, three categories of open source licenses will be discussed: "academic," "reciprocal," or "hybrid." Although there are many varieties of open source licenses in use today, both end user software and custom software components or systems that contain open source software are likely to be licensed under one of these three regimes. The specific examples described in each category below identify some of the main legal issues that licensees of open source software face. Note that for any copyright-based license grants contained in a given open source license, the fair use doctrine may apply. In addition, according to the conventions of contract law, any ambiguities in a license may be interpreted against the drafter—in these cases the licensor—regardless of extrinsic evidence of the licensor's intent.


As used on this page, the following terms will mean:

"Academic" Licenses

The term "academic" for this category of licenses comes from their genesis in academic institutions. These licenses usually allow licensees to modify the open source software, and do not require making those modifications available as open source to anyone else. This means that a software developer using open source made available under an academic license could ultimately create a proprietary product. “Academic" licenses include the BSD, MIT and Apache licenses.

Example: Berkeley Software Distribution (BSD)

Main features:

Potential Legal Issues:

For further reading on the BSD license and other academic licenses, see:

"Reciprocal" Licenses

Reciprocal licenses, which some authors may alternatively refer to as "copyleft" or "viral" licenses (depending on the writer's opinion of them), require that a licensee who distributes any open source software or derivatives thereof must license those derivatives under the same terms as the reciprocal license, but only if the licensee chooses to distribute the derivates to others. A main objective of the reciprocal license is to prevent licensees of open source from creating proprietary versions of the software, thereby decreasing the risk of “forking” that was historically the result of the privatization of UNIX.

In addition to the GNU General Public License ("GPL") discussed below, other widely used reciprocal licenses include the GNU Lesser General Public License ("LPGL"). The main distinction between the GPL and the LPGL is that the LGPL authors intend it for distribution with software "libraries." A software library is a sequence or set of source code that performs a specific function and can be "linked" to other software to create a more robust program. A software developer might write source code for a program, but will use an already-created library to add a function to the program without creating that particular function from scratch. Code embodying commonly used functions can also be fluidly created and destroyed when needed or not, or shared by multiple processes, thus sharing or conserving memory at run time. The LGPL allows dynamic linking to an open source library without a reciprocity requirement. But if the developer makes any modifications to the library, the developer has created a derivative work of the library, which requires that those changes be available under the LGPL. The LGPL does not require that the developer's proprietary code (linked to the library) be distributed as open source, but it does include other restrictions.

Example: GNU GPL 2.0

Main features:

Potential Legal Issues:

For further reading on the GPL and other reciprocal licenses, see:

"Hybrid" Licenses

The term "hybrid" is used for purposes of this discussion to describe licenses that contain elements of both academic and reciprocal approaches. Hybrid licenses allow for development of proprietary software products, so long as the specific files containing modifications of open source that was licensed under a hybrid license are, in turn, made available to all as open source.

Example: Mozilla Public License ("MPL")

Main features:

Potential Legal Issues:

For further reading on the MPL and other hybrid licenses, see:

For other web sites that compare the main license provisions of academic, reciprocal, and hybrid licenses, see:



*Fair use doctrine in brief: The fair use defense to copyright infringement provides that a copyright-protected work may be used in a reasonable manner by someone other than the copyright holder, without that copyright holder's permission. In determining whether a use is reasonable and therefore fair, courts consider a variety of factors, including the purpose and character of the use, the nature of the copyrighted work, and the effect of the use on the potential market for the copyrighted work. Because the uses mentioned as potentially fair in the Copyright Act are largely non-commercial, courts often look to see whether the use was primarily for commercial benefit, or for private commercial gain. Commercial use does not foreclose the fair use defense, however.

1 Paul H. Arne, "Open Source Software Licenses: Perspectives of the End User and the Software Developer," pp. 8-9 (2004), available at

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