Collaborative Design is a topic of particular current interest. Existing software allows a multiplicity of designers to work on the same project. What the software really allows is accessing to a part of the information of the project and changing it. Sometimes there is a hierarchical distribution of the power of change: some participants can be permitted to operate only on some limited layers of the object representation. In this case the changes they propose are to be accepted by a general manager of the design process. What is lacking in this kind of software is the explicit management on the reciprocal constraints posed by different participants. In this paper, an elementary Collaborative Design System is presented whose main concern is just the management of constraints. Each participant designs the part of the project of his/her concern instantiating objects comprised of geometric description, alphanumeric variables and constraints on both. Constraints can be of two types: absolute or defined by a range of allowed values of the constrained variable. A participant intervening later can accept the constraint, choosing a value in the permitted range, or decide to violate it. In this case the proposed violation is signalled to whom posed it.