This very post is delivered to your browser or news reader by the famous and fabulous WordPress blogging system. In my work as the developer of MarsEdit, I am exposed to countless blogging options, each with its own strengths and weaknesses. And yet, I stick with WordPress because it strikes a balance of power and ease of use which feels comfortable to me. Not to mention that Joseph Scott and others are tirelessly working to improve its API.

WordPress is licensed under the terms of the Gnu Public License (GPL) which, in a nutshell, stipulates that you are free to use the software however you like, but if you make changes and distribute those changes, then you must share those changes under the same terms. This simple, radical restriction means that you are prohibited from taking a GPL project and incorporating it with a closed-source project.

Violating The GPL

Violating the GPL is easy. All you have to do is write some code, intermingle it with some GPL code, distribute a changed copy of the original, and refuse to share your contributions. Bam! You’re toast. Assuming the original authors discover your violation and decide to pursue a resolution.

Once a violation occurs, it might be settled privately, or could escalate to legal court procedures. But the most obvious form of resolution is for the author of the changes to release their code to the public under the terms of the GPL.

Depending on how much code you “mixed” with the GPL code, this could mean only a small portion, or it could mean the entire source code of your project. This so-called “viral nature” of GPL is what scares the bejeezus out of companies, large and small, who fear the consequences of having to give up their own intellectual property to the public.

The terms of the GPL sound pretty simple at first read, but due in part to the epic consequences of a violation, there has been a great deal of debate and uncertainty about what legally constitutes a violation. Most of the debate seems to boil down to two questions:

What counts as a change to the original product? What counts as distribution of those changes?

If you can legally justify that your additions to a GPL project either don’t change or derive from the original product, or haven’t technically been distributed, then you are not subject to the restrictive terms of the license.

Take GIMP, the popular GNU-licensed image editing application. The application supports plugins, analogous to the types of plugins you might find for the commercial, closed-source application Photoshop. A savvy developer may argue that a plugin doesn’t meet the criteria of changing the original application, because the original application still runs in its unaltered condition whether the plugin is there or not.

But promoters of the GPL take the position that plugins, by nature of being loaded into the same code space as other GPL code, do constitute a modification of the original, and are therefore subject to the terms of the GPL. As far as I know this is not a question that has been well-tested in courts.

Let me take a moment to make this abundantly clear: I respect the rights of authors to license their software under whatever terms they choose, including the GPL. In my opinion, all the legal mumbo jumbo ceases to matter once the original author’s intentions are made clear. So if the author of GPL-licensed code clarifies to me that it cannot be run on Sundays, then their GPL means it cannot be run on Sundays. But this is one of the problems with the GPL: its terms are not often understood, even by the authors of GPL-licensed code.

WordPress Themes & Plugins

WordPress supports two explicit forms of extension, each of which may affect the appearance and functionality of the product. Themes tend to work as a “skin” for the appearance of a blog, while plugins tend to introduce completely new features. Since plugins in WordPress are analogous to GIMP or Photoshop plugins, it would stand to reason that they would also be covered by the terms of the GPL. But what about themes?

Themes have been controversial in the WordPress community, as a few commercial business models have sprung up to take advantage of bloggers’ desires for high quality themes at an affordable price. One approach is to distribute “free” themes that contains commercial ads. So you might stumble upon the perfect theme for your blog, only to learn that the glaring “Brought to you by Hostess Cupcakes” line near the bottom of your page cannot be removed.

But the terms of the GPL, if themes are covered, would require that end users be granted the legal right to modify and redistribute their own copy of the theme. Zap the sponsorship, reupload to your site, and you’ve got a free, high quality theme with no ugly ads.

Today, Matt Mullenweg of the WordPress project announced his lawyer-supported opinion that themes are partly covered by the GPL:

I reached out to the Software Freedom Law Center, the world’s preeminent experts on the GPL, which spent time with WordPress’s code, community, and provided us with an official legal opinion. One sentence summary: PHP in WordPress themes must be GPL, artwork and CSS may be but are not required.

If you’re starting with the understanding that WordPress itself is GPL, and WordPress plugins are GPL, then it’s not so much extra hay on the camel’s back, to also clarify that its themes are to some extent GPL. But it got me thinking again about my own blog, and about the restrictions the GPL imposes on the kinds of things I can do with the software that runs it.

GPL Stifles Participation

Now for the most controversial point of this article, where I suggest that the GPL does more to harm collaborative development than it does to help it.

For the purposes of this argument, let me reduce all the source code in the world down to three rough categories. I recognize I have omitted some classes of license here, but for the sake of argument, most projects fall into these camps:

GPL code. Changes may be distributed only in other GPL products. Liberal-licensed code. (MIT/BSD/Apache/etc). Changes may be distributed anywhere. Appropriate origin-attribution may be required. Closed-source code. May be distributed only by the copyright owner and other explicit licensees.

Now, there are a few people in the world who, for political or philosophical reasons, will only participate in a GPL project. And for comparable yet opposite reasons, there are some who will only participate in commercial, closed-source projects. But I propose that the vast majority of developers will participate in any project that is advantageous to them.

So let’s imagine a representative, run-of-the-mill developer who is working on a project that falls into each of these three camps. If this developer is not radically committed to their own project’s license, they will naturally look to outside resources in order to bolster the success of their own work.

As the developer evaluates communities to participate in, they must evaluate the legal impact such participation will have on their own project. The closed source communities are, by definition uninviting to outsiders. GPL communities are open and embracing of other GPL developers, but generally off-putting to liberal-license and closed-license developers. Only the liberal-license communities are attractive to developers from all 3 camps.

I know what some of the GPL-enthusiasts are thinking now: “leeches don’t count as community.” Many GPL developers take comfort in the fact that their hard work can’t be quietly taken and incorporated into a commercial product, without any payback of time or money to the original project. But you’re piloting an open source project, and the first step of building a community is to get people in the door. Liberal licenses? Whoo-eee do they ever get people in the door.

If you operate from the presumption that great developers love to build great projects, the first step in any successful open source project is to get as many great developers in the door as possible.

It’s Your Party

Yes, this is just me and my crazy theories. I haven’t done exhaustive research to prove that liberal-license communities thrive more than GPL communities. But the anecdotal examples are staggering. The very foundation of Mac OS X, the operating system through which I’m typing, is thanks to the liberally-licensed FreeBSD operating system.

Looking over to the right of my screen, I’m watching this sentence appear in a live web preview as I type, thanks to the WebKit project, whose liberal license makes it compatible with closed source projects such as Safari, as well as open source efforts such as Google’s Chromium project.

For years, the problem of a generic HTTP client library that runs on every major platform has been addressed by libcurl, whose liberal license has caused it to be embraced by countless companies and projects.

The popular Subversion source control system’s liberal license enabled Sofa, a commercial software business to contribute value to the community with its extremely polished, award-winning client application. Meanwhile, the newly popular distributed source control systems presents three major choices: git, Mercurial, and Bazaar. All are restricted by the GPL-license, and therefore none is likely to inspire development of a Versions-caliber client.

I’ve touched the tip of the iceberg, and yes I’ve neglected to mention some GPL success stories such as Linux, MySQL, and gcc. These communities have thrived to some extent because the passions of the GPL community are strong, but we can’t know whether their success is in spite of the restrictions their license places on participation by the broader developer community.

Speaking of GPL succeses, WordPress is itself an example of monumental success. All of its developers have something to be immensely proud of. But whenever I am reminded that WordPress is GPL, my passion for it takes a bit of a dive. I’m more comfortable with the true freedom of liberally-licensed products. If a liberally-licensed blog system of equal quality, ease of use, and popularity should appear, my loyalties to WordPress would not last long.

It’s your party, and you’re entitled to write the guest list. But take a look around the room: not as many folks as you’d hoped for? Liberally-licensed projects are booming. Speaking for myself, a developer who has been to all the parties, I’m much more likely to pass through the door that doesn’t read “GPL Only.”