One widely accepted metaphor in design research is search or, equivalently, exploration which likens design to intelligent movement through a possibly infinite space of alternatives.  In this metaphor, designers search design spaces, explore possibilities, discover new designs, and recall and adapt existing designs. We give the name design space explorers to computer programs that support exploration.  This paper describes an efficient representation of states comprising three-dimensional non-manifold solid models and of design spaces made from such states.