This paper presents a tool for assisting 3D modelling from perspective projections. The proposed system uses a projective geometry engine capable of rebuilding a 3D model from a set of lines and geometric constraints. A dialogue interface assists the user in expressing geometric constraints. The system finds out the relevant geometrical components in the sketch and automatically posts the constraints. The system is limited to rebuilding polyhedrons. If we can deduce, from a perspective sketch of a polyhedron, the planar constraints of the faces and the parallelism constraints of the edges that meet in the same vanishing points, the geometric engine will be able to build a 3D model of the polyhedron.