When implemented in a concurrent language, a window system can be concise. If its client programs connect to the window system using an interface defined in terms of communication on synchronous channels, much of the complexity of traditional eventbased interfaces can be avoided. Once that interface is specified, complex interactive programs can be assembled, not monolithically, but rather by connecting, using the same techniques, small self-contained components that each implement or modify elements of the interface. In particular, the window system itself may be run recursively to implement subwindows for multiplexed applications such as multi-file text editors. This is the software tool approach applied to windows.

The Plan 9 window system, 8 1 /2 , is a modest-sized program of novel design. It provides ASCII I/O and bitmap graphic services to both local and remote client programs by offering a multiplexed file service to those clients. It serves traditional UNIX files like /dev/tty as well as more unusual ones that provide access to the mouse and the raw screen. Bitmap graphics operations are provided by serving a file called /dev/bitblt that interprets client messages to perform raster operations. The file service that 8 1 /2 offers its clients is identical to that it uses for its own implementation, so it is fundamentally no more than a multiplexer. This architecture has some rewarding symmetries and can be implemented compactly