In part as a reaction to this distributor-driven model of copyright licensing, programmers developed what is now known popularly as "Open Source" licensing. The development of this manner of software development and licensing has been described well elsewhere and will not be repeated here. For more details on the history, read Free As In Freedom (Sam Williams, O'Reilly 2002), The Cathedral & The Bazaar (Eric S. Raymond, O'Reilly 2001), and Open Sources: Voices from the Open Source Revolution (DiBona et al., O'Reilly, 1999).
The fundamental purpose of open source licensing is to deny anybody the right to exclusively exploit a work. Typically, in order to permit their works to reach a broad audience, and, incidentally, to make some sort of living from making works, creators are required to surrender all, or substantially all, of the rights granted by copyright to those entities that are capable of distributing and thereby exploiting that work.
Because these entities, by their very nature, do not see work as work in the first instance, but rather as the source of an income stream flowing from its exploitation, they are jealous of their right to exclusive exploitation of the work. They are similarly reluctant to share any part of the value of the work with others. While the potential consumers of a literary or musical work will be limited by the costs of acquiring the work costs that are set exclusively by the person or entity that controls the right to distribute it market forces will tend to reduce prices so as to maximize returns to that person or entity. Because the marginal costs of mechanical reproduction are relatively low, selling more copies of a work (at lower prices) will generally result in a larger stream of income to the publisher.
As a result, publishers fiercely defend the copyrighted work from unauthorized distribution of copies of the work itself or creation of derivative works based on the work. In the case of artistic works, the problem of unauthorized distribution of the original work is more common. While unauthorized derivative works occasionally result in lawsuits or other disputes, the value of artistic or aesthetic works relies on their original form of expression: they are "non-dynamic." Consumers want to hear Bruce Springsteen's Born To Run and to read Dave Eggers' Heartbreaking Work of Staggering Genius; they most likely do not want to hear Dave Eggers' Born To Run or read Bruce Springsteen's Heartbreaking Work of Staggering Genius.
By contrast, software is both functional and dynamic. Each program contains code that is both functional, in the sense that it does work, and dynamic, in the sense that it can perform those functions in an entirely different context. As a result, each program that is created presents two distinct types of value. The first is its formal purpose as a database or another application. The second is a potential source of code for use in performing other functions.
 The value of work that simply inspires pleasure in the observer is self-evident. However, the fact that software essentially operates like a tool it is more like a handsaw than a sunset makes it fundamentally different than a purely aesthetic creation.
When a consumer purchases a piece of software, say, Microsoft Excel, she acquires, along with the physical copy of the software and the manual (if there are such physical copies), the right to use the software for its intended purpose in this case, as a spreadsheet program. By opening the plastic wrap on the box, the consumer becomes bound by the so-called "shrinkwrap license" under which she is bound not to copy the work (beyond the single copy made for her own use), not to make derivative works based on the work, and not to authorize anyone else to do either of these two things. The elimination of these three restrictions is the foundation of open source licensing.
 Such "shrinkwrap licenses" are provided with virtually every copy of commercial software sold today. Although such licenses do not present the formalities that people usually associate with contracts, they are generally enforced as binding contracts. Specht v. Netscape Comm. Corp., 00 Civ. 4871 (AKS), 2001 WL 755396 (S.D.N.Y. July 5, 2001). The enforceability of shrinkwrap licenses is discussed in Chapter 6.
A comparable consumer of open source licensed software is in an entirely different position. She can freely distribute (in exchange for payment or not) copies of the work because of the "open distribution" principle. She can freely modify the work and distribute those derivative works (again, whether in exchange for payment or not), because of the "open modification" principle. The only substantial limitation upon her exercise of these rights that an open source license is likely to impose is that the copies of the work that she distributes, whether the original work or her own derivative work, be themselves licensed in a manner consistent with the original license.
For example, an open source license may require that derivative works be distributed on the same terms under which the licensee was permitted access to the work under the original license. This means that those people who receive copies of these works must themselves be able to redistribute the original and to make derivative works from the original, subject only to the limitation that they allow others to do the same. This principle is called "generational limitation." This limitation may, depending on the terms of the original license, prevent open source code from "going closed" and require that users and contributors to the code abide by the communitarian values of open source.
 The term "copyleft" has been used to describe this type of restriction of redistributions of such a work and derivative works. Copyleft is described in more detail in Chapter 3. Because licensors can (and do) impose other types of limitations on second and succeeding generations of derivative works, copyleft is not the equivalent to a generational limitation but is rather one example of such a limitation.
While open source differs from the operation of traditional copyright licensing by permitting both open distribution and open modification, the removal of the second type of limitation is probably the more important one. By requiring that copyright holders both make available a user-modifiable source code for programs that they distribute and by requiring that they permit the development and distribution of derivative works, open source licensing makes possible three substantial improvements over traditional proprietary commercial software licensing models.
The first, and perhaps the greatest, of these benefits is innovation. It is now well-demonstrated that programmers are willing to contribute to open source projects for no reward other than that of making a program more useful. Open source works. The more programmers that can contribute to a given work, the more value that work is likely to have.
 This may be another meaningful distinction between software and aesthetic works. Aesthetic works may benefit less from contributions from many participants.
The second benefit is reliability. Many programmers means many people who are available to debug a given program. Moreover, the benefit is not simply one of numbers. A knowledgeable user, who has witnessed firsthand the limitations of a particular application or the effects of a bug on a program's operation, is generally in a better position to address that limitation or to fix a given bug than an employee of the creator of the original software. Such a user almost certainly has a greater incentive to correct such a shortcoming in a given piece of code than a software publisher, where suggestions to make such corrections must compete not only with other perhaps more pressing corrections, but also with the publisher's own financial or organizational limitations.
The third benefit is longevity. When commercially licensed software goes "out of print" and is no longer supported by its publisher, there is generally no way that software can be updated or adapted to new uses. Such software comes to an evolutionary dead end. By contrast, open source licensed software can fall into disuse for some period but still be revived, adapted, or rewritten by a subsequent user who finds a use for it a use that may be completely different from the use originally intended.