Abstract: The production of most mass-market software can be grouped roughly according to free and proprietary development models. These models differ greatly from one another, and their associated licenses tend to insist that new software inherit the characteristics of older software from which it may be derived. Thus the success of one model or another can become self-perpetuating, as older free software is incorporated into later free software and proprietary software is embedded within successive proprietary versions. The competition between the two models is fierce, and the battle between them is no longer simply confined to the market. Claims of improper use of proprietary code within the free GNU/Linux operating system have resulted in multi-billion dollar litigation. This article explains the ways in which free and proprietary software are at odds, and offers a framework by which to assess their value - a prerequisite to determining the extent to which the legal system should take more than a passing, mechanical interest in the doctrinal claims now being pressed against GNU/Linux specifically and free software generally.