'CBuilder' is a separate RSCAD module used for building Draft component icons. In addition to an advanced graphical editor to create and edit the icon, it allows for customizing all aspects of the model, including the user-entered parameters and the simulation code behind the model. It has adopted many of the conventions of other popular drawing software, in an effort to make the interface intuitive and easy to use. Because the creation of a model is a fairly complex process, the CBuilder module itself is fairly complex, with several different components to it. The data associated with the model is entered in the 'Parameters' tab, and then this data can be used in 'c' type expressions when defining the model's graphics, computations and nodes. The user writes the executable code for the model in standard 'c', in a file that has been divided into initialization logic and the simulation code. Emphasis has been put on developing the interface so that it presents a consistent approach to defining the various aspects of the custom model. The CBuilder program is now the main development tool of our in-house developers, and is continuously being upgraded and enhanced.