The design tool cTc has been developed to support the construction and refinement of formal specifications of distributed software systems. It concentrates on the design by stepwise refinement, where refinement steps correspond to the integration of predefined process patterns. cTc processes modules written in the compositional TLA specification style cTLA. It applies the cTLA specification operations and generates the resulting specification modules. We outline the background of cTc and give an overview on its functionality and architecture.