"One prominent example of free, open-source software is the Linux operating system, a derivative of the Unix operating system written by AT&T in the 1960s and now available without cost. (Unix® is a trademark of The Open Group, but the source code to many variants of AT&T's work is freely available.) Linux is one of many modern derivatives of Unix — which is not itself under the GPL. Thus Apple Computer, which uses the Berkeley Software Distribution variant of Unix as the foundation for the Mac OS X operating system, is entitled to charge for its software. Linux, initially the work of Linus Torvalds, is maintained by a large open-source community. International Business Machines offers Linux with many of its servers, or customers can install it themselves. IBM has contributed code to the Linux project and furnishes this derivative work to anyone else with an interest. Red Hat, Inc., sells media (such as DVDs), manuals, and support for the installation and maintenance of Linux. The GPL covers only the software; people are free to charge for the physical media on which it comes and for assistance in making it work."