Abstract: In 1989, Gaggioli, Sama and Qian published a series of 10 "second law guidelines" for design and process engineers, which were discussed for some time in thermodynamic conferences and papers under the name of "the Gaggioli-Sama rules", though some of them had been previously published by Sama between 1980 and 1983. These guidelines are in essence a list of "second law errors" to avoid in the design of energy conversion systems. The list was later rearranged several times, and its latest version, containing 21 rules, was published by Dominick Sama and Jan Szargut in 1995. Ever since, these guidelines came to be known as "the Sama-Szargut rules" The rules are sound, well-formulated, insightful, and reflect a thermodynamicist's idea that the "best design" is the one that minimizes the overall irreversibility in the process or plant under consideration. Characteristically, the idea of "optimality" is completely absent from all published formulations, the insistence being on the extensive inclusion of second law reasoning into design decisions. One would therefore expect that most, if not all, of the rules be routinely implemented in new designs and in retrofit projects. It turns out, that the contrary is rather true: even a superficial survey of some of the current most common energy conversion installations shows that most of the rules are actually disregarded in practice. This paper contends that the reason for this apparently irrational behavior is the neglection in the engineering design decision of the real cost of installation, operation and dismantling of a plant. It is argued that, if the production cost is assessed in terms of equivalent primary exergy and a proper exergy accounting for the externalities is introduced, the Sama-Szargut rules can be directly interpreted in this sense, and abidance by the rules results in the reduction of the resource cost for any given objective.