A fuzzy sets based on structure for classification systems is proposed in this paper.  The basic idea is to use “windows” to represent the constraints on the possible values variables may take. The formalism is very simple, however, this simplicity makes it attractive in the development of knowledge based systems.  The most salient features of this structure include the possibility of developing composite solutions, searching for lacunae, and creating a case based representation of knowledge with an avenue for modeling learning.