My old high school's prom was this past weekend. The photos have started to come in on MySpace and Facebook. It looked like everybody had a good time. And of course, it got me thinking about my own prom, which was last year. I could go on for a while about how great it was. I could mention more details that I didn't add to the original post. For example, one of the songs I remember dancing to was "Hey Ya!" Everyone went crazy immediately upon hearing the count-in. I could also mention how the only regret is that I didn't get the nerve to talk to Jannike sooner. But I won't. I just hope the Class of '08 had as good of a time at their prom as I did at mine.

Anyway, what I really wanted to talk about today was copyleft. What's that, you ask? Before we answer that question, we have to ask: What is copyright? Mr. Webster says:

For example, the above quotation is from the 1913 edition of Webster's dictionary, which is no longer protected by copyright. As such, I can quote as much of it as I want in this blog post, and everyone in general can do anything with it that they please. If it had been copyrighted, I wouldn't have been able to quote very much without first requesting permission from the copyright holder and likely paying a large fee.

Over the years, copyright protection has become much more widespread in several ways. First, it is much easier for works to be copyrighted. Instead of having to file a form with the U.S. Copyright Office and include a copyright notice in your work, anything you create in a tangible form is instantly copyrighted whether you include a notice or not. Second, the control that copyright holders have over their work has been extended to include not only copying but also adapting, performing, translating... basically everything but quoting brief passages. Third, copyright terms have been extended drastically, from 56 years in 1975 to 120 years or more in many cases today. The effect of all this is that it's getting harder for people today to follow in the footsteps of generations of innovators in science, business, and entertainment, who advanced our culture by building upon material that had little or no copyright protection.

Enter copyleft. First devised for computer software in the 1970s, "copyleft" refers to any form of copyright licensing in which the copyright holder revokes some of his exclusive rights to the work and requires anyone who modifies the work to do the same. So you can be sure than any work that is covered by copyleft will always be free for other people to use, no matter how it is changed. (Until the copyright expires, at least.)

Maybe you've heard of the GPL, a copyleft software license. It's the license under which Linux-based operating systems are distributed. The GPL and similar licenses allow a group of people to collaborate on software, and that's part of what has made Linux so popular. Software development under the GPL advances rapidly because it allows people to adapt and extend other GPL-licensed programs. And there are many thousands of them. It's exciting stuff. What's more exciting (if you can believe it) is the recently released version 3 of the GPL, which closes a lot of loopholes that companies have discovered in the previous version in recent years.

Recently, it's become easy for people to apply copyleft to other works. This is largely due to Creative Commons, an organization that provides licenses for people to "easily mark their creative work with the freedoms they want it to carry." These licenses can apply to most anything you create, whether it's a web site, a book, a painting, a song, or something else entirely. Say you took a photo and posted it on the Internet, and you'll let anyone use it for any purpose as long as they give you credit. Just say it's licensed under a Creative Commons Attribution license. Then, if somebody uses your photo without attributing you, they've violated the legalese of the license, and you can sue them. Other licenses prohibit making derivative works and/or using the work commercially. There are fun mix-and-match combinations to suit just about every need.

You may notice that these requirements themselves do not constitute copyleft; that is, someone who modifies someone else's work and just gives credit or uses it non-commercially can declare "All rights reserved" in their new version. But several Creative Commons licenses require users of the work to "share alike." I used to have this site licensed under an Attribution-NonCommercial-ShareAlike license, meaning that anybody could use my work for noncommercial purposes if they gave me credit and released their new version under the same license. Then, I got some requests to use some of my images for commercial purposes. I had to give them permission explicitly because the license forbids it in general. But I thought about it, and I realized that I wouldn't really mind if my works were used for commercial purposes. As long as they remain under copyleft ("ShareAlike"), it doesn't harm me or the general public. So recently, I re-licensed the content of this site under an Attribution-ShareAlike license. (Check out the sidebar and the footer to see those links you've never noticed!)

But I have to admit, folks: I haven't always been this attentive to copyright issues. I wrote The Dvorak Keyboard and You almost 6 years ago, when I was 13. When I needed an image of the Dvorak keyboard layout to illustrate the article, I searched the web and found high-resolution scan that someone had made from an MS-DOS manual. I prettified it, and other people copied it when they linked to me. That's no way to respect someone's copyright, even if it is Microsoft's. Yesterday, I finally decided to correct this indiscretion. I searched Flickr for a diagram of a keyboard that I would be able to use under the terms of the Attribution-ShareAlike license. I found one with an Attribution license, modified it for my own needs, gave the author credit according to the license, and relicensed the new work under the Attribution-ShareAlike license. (This is allowed because the original author is still being attributed and has not restricted the right of others to require sharing alike down the line. At least, that's what this table says.) Now, if someone comes along wanting to use my modified image, they'll have to give me and the original author credit and allow others the freedom to modify it for their own purposes. That's copyleft.

Lawrence Lessig, the founder of Creative Commons, has some interesting things to say about copyright in his book Free Culture, which I'm currently reading. It's pretty cool stuff, but I'll hold off on it until I've finished reading it. Besides, this has been enough for one post. And now, the links.

Here's an Animated History of the NYC Subway. It's an animated map that shows each line in the order it was built.

Math humor: 36 Methods of Mathematical Proof. This reminded me so much of my differential equations professor (what with his ingenious math tricks and all) that I sent him the link. I'm not sure whether he liked it or not.

Firefox Logo Spied in Deep Space? Okay, the image is a little modified from the original, but there's still a resemblance if you look at it the right way. At least it doesn't look like a woman wearing long robes or a guy with a beard.