(UC Berkeley), found itself involved in a license-related lawsuit filed by AT&T. At around the same time, the GNU project, which developed a UNIX-like free software not infringing on the AT&T license, was nearing completion-the "jigsaw puzzle"-with the exception of the kernel. Linux, a free UNIX-style software kernel with open source code free from the AT&T license made its appearance as the desired kernel becoming the "last piece" of the puzzle. A UNIX-type operating system is an aggregate of single function programs called tools, which are easily substitutable. Under these conditions, the free software system, noted for being "UNIX running on a PC," was completed in a short span of time. Essential to the success of Linux was its miraculous release timing. It goes without saying that this phenomenon is unlikely to be repeated even under ideal conditions of it being free of charge and open source.