For most of the 2000s, copyleft licenses (in particular the GPLv2) were the most popular choice for new open source projects. In the last few years, developers and companies seem to be trending away from the GPL in favor of permissive licenses for open source projects. What's behind that, does it impact your business and what licenses should you choose for new projects? Let's take a look.The GPL is in decline, sort of. As Matthew Aslett reported last year, the number of projects using the GPL family has increased in real terms. However, the usage of the GPL as a percent of all open source projects is in decline. According to Aslett, in 2008 the GPL family was 70 percent of licenses. As of December of 2011, it was 57 percent. Clearly, there is a trend at least for now towards permissive licenses. Defining Permissive Licenses and CopyleftFor those who don't spend a lot of time thinking about open source licensing, let's look at the differences between permissive licenses and copyleft. Copyleft licenses look to protect developer and user rights. As the Free Software Foundation puts it, the GPL protects the "four freedoms":\nThe freedom to run the program, for any purpose.\nThe freedom to study how the program works and change it so it does your computing as you wish.\nThe freedom to redistribute copies so you can help your neighbor.\nThe freedom to distribute copies of your modified versions to others.\nYou can do all of those things with a permissively licensed project, but what the permissive licenses do not do is carry any requirement that the recipient of the software pass those freedoms on to their users. For example, companies that work with the Linux kernel and distribute it are obligated to make source available to their users. This is frequently a problem when companies distribute modified GPL'ed software like the Linux kernel or Busybox in embedded devices and neglect to make source available. Why Companies Like Permissive LicensesOn the other hand, this is not a problem for companies that are distributing software under a permissive license. There are some requirements around copyright notices for some permissive licenses, but they don't require companies to distribute modifications. In the eyes of some companies and developers, the permissive licenses have several practical advantages. First, they allow companies to create proprietary versions of software if they choose. Some companies want the option to distribute a proprietary version and don't like the idea of having to use the GPL for derivative works. In some cases, it's simply a matter of convenience. The GPL doesn't always mix well with other open source licenses. Dealing with the licensing questions that come along with GPL software can be a headache for companies. Compliance is also a bit of a headache for companies that are not adept at dealing with open source. Ensuring that your company distributes the source and complies fully with the GPL, including making sure that if the GPL is combined with other works that it is license-compatible, can require a fair amount of attention. Finally, there's a philosophical argument to consider. Some developers complain that the GPL is restrictive, and prefer a license that is truly free and allows any re-use. For those developers, the most important thing is that the software is used. They don't really care if the downstream distributor of their code passes on the "four freedoms" to end users. Why You Might Prefer CopyleftThe counter to this is that if you received those freedoms, you should pass them on to your users as well. If the goal in producing software is to provide free software to users, then copyleft is really only way to do that.It also can be used competitively. Some companies look to copyleft licenses to protect their investment in software. For example, if a company elects to open source its core product, odds are that a copyleft license is going to be a much better choice. This ensures that any competitors that want to make use of the software also have to contribute changes back. Open Source All The StuffTom Preston-Werner of GitHub talks about GitHub's policy around open source. It can be best described as "open source (almost) everything." Preston-Werner makes a strong case for open sourcing as much software as possible, because it provides a number of benefits: \nGreat advertising (draws attention to your company)\nForce multiplier (more users, more contributions, faster improvements)\nGreat for attracting and evaluating talent \nIt's the right thing to do\nSo what to they use? Preston-Werner says they prefer the MIT License, because it's short, offers "enough protection" against law suits, and "everyone understands the legal implications." More and more companies seem to be thinking like GitHub. What This Means for Your BusinessFor the last six months, I've been paying a lot of attention to software for cloud services, big data and Web development. When companies such as Twitter, Facebook or Rackspace release software it's usually under a permissive license. A lot of the "action" right now is in the Hadoop space, for example. Almost all, if not all, of the software in the Hadoop ecosystem is under the Apache License. (Not surprising, since Hadoop is an Apache project.)For companies that merely consume open source software, the trend is fairly neutral. Lots of interesting software continues to be developed as open source and is available to your business. Philosophically, I prefer the copyleft licenses. However, practically, I can see the rationale for businesses standardizing on permissive licenses. For companies that participate and distribute open source, permissive licenses mean fewer problems with compliance issues. There are fewer requirements related to distributing source, and fewer questions around license compatibility. \nJoe 'Zonker' Brockmeier is a freelance writer and editor with more than 10 years covering IT. Formerly the openSUSE Community Manager for Novell, Brockmeier is a longtime free and open source software advocate. He has written for many publications, including Linux Magazine, Sys Admin, Linux Pro Magazine, IBM developerWorks, Linux.com, CIO.com, Linux Weekly News, ZDNet and many others.