Twenty years after the general adoption of overlapping windows and the desktop metaphor, modern window systems differ mainly in minor details such as window decorations or mouse and keyboard bindings. While a number of innovative window management techniques have been proposed, few of them have been evaluated and fewer have made their way into real systems. We believe that one reason for this is that most of the proposed techniques have been designed using a low fidelity approach and were never made properly available. In this paper, we present Metisse, a fully functional window system specifically created to facilitate the design, the implementation and the evaluation of innovative window management techniques. We describe the architecture of the system, some of its implementation details and present several examples that illustrate its potential.