Copyleft is a concept popular with those operating in the world of tech and software development, and is, basically, the opposite of copyright. Under a copyleft, source code, and any developments based on that originaly code, must remain “open source.”

Q. So, this is another topic in the concept we started talking about yesterday, so important in the digital world: the business model of “free”.
A. Correct. There’s a very vibrant community behind this idea, especially parts of the software development world. Pretty obviously, the point of “copyleft” is that’s the opposite of copyright: instead of reserving rights to your work, you give them away. A user can take, modify, reproduce and distribute your software code without paying.

Q. Well, before we get to how you could possibly get a business model out of that, let’s ask: why? What’s the idea?
A. The idea really revolves around collaborative development. One big point of the copyleft movement, aka “open source”, is that if you take my source code under a copyleft, and figure out some new improvement or addition, you have to license that to the world under the same conditions that you got my code. So it lays the foundation for, essentially, a worldwide development team. All improvements are shared.

Q. OK, that’s cool, but this is a business show. So how does anyone make money?
A. The perfect example is Red Hat, the Linux provider… which just passed $1bn in revenues. Red Hat Linux itself is a free and open source version of Unix (actually written by my friend Ken Thompson when he was at Bell Labs, one of the great accomplishments in Comp Sci history). Red Hat distributes free software, but provides all sorts of consulting and support services for large corporations that deploy it. Meantime, an army of outside developers keep pushing the software forward, and Red Hat doesn’t have to pay them! So, that’s how you make money off of “free”.

Q. And how about other examples?
A. My favorite one is actually a comic strip, XKCD. That’s published under the “Creative Commons” license, Lawrence Lessig’s version of copyleft for the creative community. The author publishes a comic, for free, on the internet; and lets anyone use it, republish, and remix it. But he has a very nice business selling all sorts of paraphenallia around the content of the strip: clothing, posters, autographed reprints of the most popular ones. So, again: free as a business model.