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In this paper we attempt to present a very general approach to the study of structures (somehow) defined on a set X by a family of maps d: X X R^+. It will be shown how the assignment of a preorder _ on a set of families of maps from X X into R^+ defines a structure on X. The structures obtained in this way will be called real structures. For real structures on two different sets we study when they are of the same type. An answer to this question will allow to introduce the notion of morphism, and then to give the definition of the initial real structure with respect to a family of maps, and so also the definition of product real structure. Various examples of preorders, and hence of real structures, will be exhibited and discussed. A few examples of morphisms will be proposed.
Tullio Valent (Wed,) studied this question.