SCO's lawsuit claims that IBM broke its contract by allowing parts of SCO's Unix V source code, licensed to IBM for use in AIX, to be used in the rival Linux operating system kernel.

But the history of Unix makes what SCO owns - against what is open source - a complex matter. Unix originated in AT&T's Bell Labs, later Unix System Labs.

Among others, the University of California at Berkeley produced open source code, which was subsequently merged into Unix.

Code additions also came from the most recent Unix licence owners, Novell and SCO, while companies like IBM wrote their own code - and Linux was developed separately.

No doubt the courts will help resolve this complexity. But the outcome will be crucial to vendors and users alike, and a litmus test for the very future of open source. Follow all the developments in our Special Report.