This paper presents the main structure of the Bblocks (Building Blocks) software. Bblocks was a joint research project developed by the Research Centre for Energy Resources and Consumption, CIRCE, directed by A. Valero, and sponsored by The International Centre for Water and Energy Systems, ICWES, directed by Dr. D. Al Gobaisi. Bblocks was conceived for the thermodynamic and thermoeconomic analysis of dual-purpose power and desalination plants. Once a plant configuration was designed by means of its guided graphical interface, its steady-state performance could be easily calculated. Sensitivity analysis and thermoeconomic analysis of the configuration could be further performed, if desired. The end objective of Bblocks is then to search the optimum dual plant configuration in terms of energy efficiency, with the help of a user-friendly software tool. Prof. Y. El-Sayed also actively collaborated in the design phase of Bblocks. His visionary research analysis of 27 feasible configurations to provide power (and water), including its own thermoeconomic optimization procedure, was a good start up to BBlocks. Moreover, some costing and modelling equations (including part load behaviour) developed in this analysis were included in Bblocks. He also visualized the need of a friendly-user interface that could allow a simple layout of any new configuration and its consequent analysis, taking into account the increasing capabilities of present computers.