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Publishers of open source software ordinarily do more than simply provide copies of both the source code and the object code when they distribute computer programs to the public. In addition, they establish the terms of use of the software by means of a license. A license is a contract through which the publisher allows recipients to use and modify the software, subject to certain conditions specified in the license. For example, the license might require that anyone who redistributes the software also make the source code of that software publicly available. A contract that provides users with a sufficient set of privileges to access and modify the software's source code is deemed an open source license.

The practice of preserving the rights of software users through a set of license provisions is sometimes called "copylefting." This term is a play on words on the term "copyright."[1] Under this system, the copyright holder licenses the recipient of a copy of the software. The license permits the redistribution of further copies of the software — including software containing modifications — under the condition that those copies are subject to the same license. This legal framework ensures that derivatives of the licensed work remain open. If the licensee fails to distribute derivative works under the same license, then he may face legal consequences. In particular, the licensor could terminate the license, leaving the licensee without permission to copy, distribute, or modify the software.[2]

Although no official definition specifies which software licenses qualify as open source licenses, an organization called the "Open Source Initiative," or OSI, has promulgated a widely followed set of standards.[3] OSI describes itself as a "non-profit corporation dedicated to managing and promoting the Open Source Definition"[4] through a certification program that it administers.[5] To satisfy the Open Source Definition, the license must satisfy certain conditions, including:

  1. The publisher must provide both object and source code.
  2. The publisher must allow modification and redistribution of the code (with or without modifications by the licensee).
  3. The publisher must not limit distribution to certain fields of endeavor or products, or limit its use with other free software.[6]

A number of different groups have promulgated a variety of open source licenses that OSI has certified as compliant with the "Open Source Definition." Among these is the General Public License or GPL.[7] As compared to the Open Source Definition, the GPL imposes additional restrictions upon software publishers. As a result, a license may fulfill the conditions of the Open Source Definition but not qualify as a GPL.

According to its sponsor, the Free Software Foundation, the GPL guarantees computer users the following "four freedoms":

  1. The freedom to run the program, for any purpose.
  2. The freedom to study how the program works, and adapt it to your needs.
  3. The freedom to redistribute copies so you can help your neighbor.
  4. The freedom to improve the program, and release your improvements to the public, so that the whole community benefits."[8]

It is important to note that, in this context, the term "freedom" does not mean that the software has to be sold at no charge. Rather, it refers to permissible user activities, and in particular the principle that software should be openly available in all of its current and future forms to all those desiring to learn or benefit from it."

The chief distinction between the Open Source Definition and the GPL is that the Open Source Definition effectively allows users to appropriate privately any modifications that they make.[9] For example, under the Open Source Definition, a user could refuse to disclose publicly the source code of any programs that include the user's modifications. The licensee could also claim intellectual property rights in any modifications they introduced into the software." Of course, in such cases, the licensee's software would not qualify as open source. In contrast, the GPL requires that the source code be kept open to the public, even if the recipient of the software made changes. Also, under the GPL, the licensee cannot restrict the ability of others to build upon any modifications that the licensee made."

For example, a computer programmer may use software under a license that minimally complies with the Open Source Definition. Suppose further that the programmer modifies the software, and then licenses the object code of the modified version with the additional restriction that no licensee could copy the modified version's object code. This practice is acceptable under the Open Source Definition. This practice would not comply with the Free Software Foundation's GPL, however.


  1. See Free Software Foundation, "What is Copyleft?" (full-text).
  2. Dennis M. Kennedy, "A Primer on Open Source Licensing Legal Issues: Copyright, Copyleft and the Future" (full-text).
  3. See Open Source Initiative, Introductory Statement (full-text).
  4. See Open Source Initiative, The Open Source Definition, Version 1.9 (2004) (full-text).
  5. See Open Source Initiative, OSI Certification Mark and Program (2004) (full-text).
  6. See Open Source Initiative, The Open Source Definition, Version 1.9 (2004) (full-text).
  7. See Terry J. Ilardi, "Mass Licensing," 747 Patents & High Technology Licensing 187 (PLI 2003).
  8. Free Software Foundation, "The Free Software Definition" (2004) (full-text).
  9. Daniel B. Ravicher, "Facilitating Collaborative Software Development: The Enforceability of Mass-Market Public Software Licenses," 5 Virginia J.L. & Technology 11 (2000).