The GNU GPL (General Public License) means you're free to download the source code of the Linux kernel, modify it, recompile it, repackage it, and even make it part of your own products if you want to. There's just one catch: Don't expect to call it Linux -- that is, not without paying up.

The word Linux means something to most people. Linux is free software. It's an operating system. It's a competitor to Windows. But when you look closer, the actual definition of what Linux really is can get a little murky.

For example, the Free Software Foundation (FSF) is quick to point out that when most people say Linux, they're really talking about a combination of the Linux kernel plus a lot of tools and libraries from the GNU Project. The FSF would therefore prefer you call it "GNU/Linux."

Then you have Red Hat Enterprise Linux -- a commercial product you can't get without a license from Red Hat. You have Suse Linux, and Slackware Linux, and Gentoo Linux -- all slightly different. All still Linux? What about those Linksys cable/DSL routers that run on an embedded Linux kernel? Can Linksys call those "Linux routers"?

Just who is allowed to call a product or service Linux, anyway?

Linus Torvalds has an answer for that: Nobody. Not without his say-so.

The term "Linux" is a trademark and Torvalds owns it. His assignee, an organization called the Linux Mark Institute (LMI), is empowered to collect licensing fees from companies and individuals who want to use the word commercially. And if all this seems surprising on the surface, the story of how Torvalds acquired the Linux trademark is testament to just how important ownership of the mark really is.

Torvalds didn't plan on gaining trademark protection for the word "Linux" when he began work on his OS, but by 1996 he started wishing he had. That's when William R. Della Croce Jr. of Boston first started demanding 10 percent royalties on sales from Linux vendors, based on a trademark claim he had filed in 1994. The Linux kernel was still free software, but according to Della Croce, the name itself was his property.

The Linux community was quick to rally to the cause. When the dust had cleared from the resulting lawsuit, Della Croce's claim was nullified and the Linux trademark was assigned to Torvalds for safekeeping.

But the story doesn't end there. It can't. To maintain a trademark, you're required to defend it. That means explicitly authorizing those who are allowed to use it, as well as litigating those who use it fraudulently. Both cost money. So LMI maintains a fairly liberal usage policy for the mark and recently has begun charging licensing fees on a modest sliding scale from $200 to $5,000, all of which go to the care and feeding of the trademark.

It all sounds so corporate, so un-open source. But really it's not. In fact, a trademarked Linux "brand" may have been inevitable. If the open source community is to maintain equal footing with the commercial entities that stand to profit from Linux, it needs to play by the same rules they do. That means using the same tools they do, including software licenses, trademarks, and -- if Open Source Development Labs has its way -- even patents. If it helps the Linux developer community gain a seat at the grown-ups' table, $200 per license is a pittance.