This report describes the design and implementation of a constraint based environment for modeling the structural description of physical sys­tems in qualitative space. The variables and constraints define the structural description and the constraint propagation derives the behavioral descriptions. The constraint propagation finds an interval value for each variable by shrinking the initial interval values of the variables, such that all the solutions are captured by the final values of the variables. During the propagation of constraints, justifications are built for each variable so to give an explanation for a behavior of the system. The constraint sys­tem is incorporated with an Assumption-based Truth Maintenance System (ATMS) to avoid recomputation.