**6.7 Techniques of modelling**

There are three main techniques, common in CAD systems for the construction of threedimensional geometries: i) primitive geometries ii) intersecting geometries iii) domain-based wireframe geometries. The latter will be the most used, in this case the objects are constructed using geometric information in wireframe, and may be straight lines, curves, arcs, points or a combination thereof. The software uses this field and applies the function selected by the user, which are: operation of revolution or extrusion of a profile on a directional vector; extrusion of a profile on another profile directional; interpolation between curves; migration between cross sections. More complex surfaces, which require patches with three or five boundaries, are more complex representation. These surfaces are found in regions with corners at the intersection of surfaces, as well as the junction of three Fillets.
