Abstract: The aim of this paper is to clarify the concepts of exergy and anergy and those related ones, laying stress on the distinction between anergy and dead state energy and unavailabe energy. Based on such clarification, a new guide to energy saving is suggested. The whole paper is composed of two parts. The first part, under the heading 'Fundamentals', describes the double constraints on energy transfer and conversion. Under the double constraints, the prosess energy of the composite system is divided into four subsets: exergy, anergy, unavailable work and unavailable heat. The authors defined 'dead state energy' in addition and pointed out that it does not belong to 'process energy'. The second part, under the heading. On Availability, demonstrates the function of anergy in energy-using processes, thus, gives 'Availability' a new meaning. And then, a new guide, named ,Double Targets', to energy saving designs is suggested.