Signal constellation design is a major subtask of constructing an efficient communication system; it essentially entails trading-off error frequency against information throughput, a chief occupation of modem designers. We propose and implement an interactive tool for designing and simulating arbitrary signal constellations. 'To construct the user interface we have utilized Interface Builder, a new interactive tool that greatly facilitates the synthesis of arbitrary user interfaces through an object-oriented methodology. Using the Interface Builder package and the Signal Constellation Design Tool as the target prototype, we show how an order-of-magnitude improvement can be achieved in the effort required to produce a complex user interface. Our secondary goal is to try to dispel some of the mystique surrounding user interface synthesis on state-of-the-art workstations by describing in detail the construction of an interactive tool for computer-assisted learning.