The GPL and MPL both have had symbolic as well as practical impacts in the world of software development. Those effects of these licenses, beyond their strict terms, are described next.
3.4.1 The GPL License and the Free Software Philosophy
The impact of the GPL, and its offshoot, the LGPL, on the development of software cannot be overstated. The GPL project that grew up with the license, the GNU/Linux constellation of applications, better known simply as Linux, has seen its acceptance by users grow steadily from the early 1990s to the point where it now poses the only significant competitor to the Windows operating system.
 For a discussion of this nomenclature, see the essay "What's in a Name?" contained in Richard M. Stallman, Free Software: Free Society (Free Software Foundation, 2002) (p. 51 and following).
This success, depending on your point of view, arises either because of, or despite, the fact that the GPL bars any development of software from GPL-licensed software that is not itself GPL licensed. The GPL seems to embody the maxim that "Freedom in a commons brings ruin to all." By requiring that all contributions to GPL projects be themselves GPL licensed, the GPL ensures not only that these contributions are available to other programmers (or at least those programmers willing to work within the GPL framework) but also encourages contributions from those programmers to whom it is important that their contributions be made, and remain, "free," as that term is used in the GPL.
 Quotation from Garett Hardin is taken from William Vollman, Rising Up and Rising Down (McSweeneys 2003) (Vol. III, p. 219). "Freedom," as the term is used in the quotation, is the absence of rules: the GPL itself is an embodiment of the principle that certain types of freedom require rules in order to be preserved.
The existence of such programmers is by now beyond dispute. Based on the success of the GNU/Linux project alone, the free software project has succeeded. Part of this success is due to the fact that the GPL has as important a symbolic purpose as a practical one. The restrictions of the GPL (and the LGPL) have greatly shaped the nature of development of software that is GPL-licensed. Programmers, by and large, respect the GPL and conform their behavior accordingly. The development of projects under the GPL depends on each participant adhering to the terms of the license by making his or her own contribution available to the community of developers. This adherence has resulted in the great success enjoyed by GPL-licensed projects.
But the GPL has an equally important aspirational purpose. Given that the GPL is often viewed as the "purest" form of licensing in nurturing and encouraging open development of software, development under this license has drawn programmers who take seriously the larger concept of open software development. This has had results beyond those caused by the terms of the license itself.
Contrary to the beliefs of some, the GPL does not require that software running on a GPL-licensed operating system be licensed under the GPL. Similarly, the GPL does not require that only GPL-licensed programs be distributed as part of a distribution containing GPL-licensed code. For example, many of the GNU/Linux distributions, including those of Caldera, a significant early GNU/Linux distributor, included both the GPL-licensed Linux operating system and proprietary licensed code. Caldera paid royalties (as required by the terms of the proprietary software) on proprietary software that was distributed on the same CD-ROM as the operating system. Because the proprietary software did not compile with the operating system, this was perfectly consistent with the terms of the GPL. Purchasers of the Caldera distribution were free to install the GNU/Linux operating system and to install the proprietary software from the same CD-ROM on the same computer.
Despite the fact that such distributions are completely consistent with the GPL, some programmers and distributors have disfavored such distributions, on the grounds that such distribution is inconsistent with the spirit of free software development. A well-known distributor of GNU/Linux software, the Debian project, for example, has taken the position that every piece of software distributed as part of a GNU/Linux distribution should be licensed under a free software license. This view, held by many members of the free software community, has significantly influenced the development of software under the GPL.
 See http://www.debian.org/social_contract for more details.
Taking just one example, a substantial movement arose to counter the K Desktop Environment (or KDE), a graphic user interface (GUI) frontend on the GNU/Linux operating system, which some perceived as an encroachment on the free software philosophy. Beginning in 1996, a programmer named Mattias Ettrich started to develop KDE based on the Qt Toolkit, a non-GPL licensed program written and owned by Trolltech. While the Qt Toolkit was available without cost, and while its source code did eventually become available, there was a strong counterreaction to the development of such a potentially critical piece of software under a non-GPL license. As a result, a separate team of programmers began to develop the GNU Network Object Model Environment or GNOME, announced in August, 1997. As a result, both the KDE and the GNOME programs, both well-supported by developers and applications, are growing and thriving today.
 For a more thorough description of this episode, see Chapter 15, "Trolls Versus Gnomes" in rebel code: inside linux and the open source revolution, Glyn Moody (Perseus Publishing, 2001).
 The Q Public License is discussed in Chapter 4.
 As noted in Chapter 4, Trolltech eventually cross-licensed the Qt Toolkit under the GPL after receiving substantial pressure to do so from the free software community.
Conversely, the development of free software projects is not determined, or even necessarily shaped, by the terms of the GPL. While the GPL encourages a certain type of development, it does not mandate any particular type of development structure. Indeed, it invites many different approaches to development. As described in Chapter 7, the initial components of the GNU/Linux operating system the GNU C Compiler and Emacs, among others were developed under a very different model of software development than the Linux kernel. Moreover, as also described in Chapter 7, even for projects licensed under non-GPL licenses, there are significant advantages to maintaining an "open development" model in which code is kept available to the open source community and not developed (as is permitted) under proprietary licenses.
While its terms may provide the foundation for free software development, the GPL is also a potent symbol of a much larger, and more important, idea of how software (or any other work) should be made and maintained. The success of this license has been driven as much by the ideals that it represents as by strict application of its legal terms.
3.4.2 The Mozilla Public License: Circumstances and Opportunities
As already described, the MPL was the result of a decision by Netscape Communications one of the first Internet companies to open source license the code to its Netscape Communicator software in January, 1998. At that time, Netscape was in an intense competition with Microsoft, whose rival web browser, Internet Explorer, had the advantage of its close association with Microsoft's dominant Windows operating system. The MPL was an attempt to get some of the benefit of open source development into a program developed under a proprietary license.
The initial announcement of the "opening" of the Communicator code was greeted with great enthusiasm (and certainly boosted public perception of open source software), although Netscape's own economic condition and its eventual absorption into America Online caused problems for the project. A thorough rewriting and a focus on standards-compliance have created a strong Mozilla culture, in many ways independent of its roots in Netscape. Open sourcing the project by itself didn't reverse Netscape's fortunes, but it has been a key source of continued innovation in the web browser market. The continued vitality of this project was demonstrated on June 27, 2004, with the release of the FireFox web browser by the Mozilla Foundation.
The MPL itself has thrived as an open source license. The well-constructed, well-written MPL has certainly found a niche: only the BSD, GPL, and LGPL Licenses are associated with substantially more projects than the MPL. The MPL has also been used as the base for a number of other Open Source Initiative-certified licenses, including the Apple Public License, the Nokia Open Source License, and the Sun Public License.
As can be seen from the examples of the GPL and the MPL, the success of licenses is a factor less of the terms or the wording of those licenses than of the ideas that they represent. Powerful, meaningful ideas draw minds, and the success of open source and free software licensing is the result of the minds that such ideas can draw.