I am mechanical engineer and I use parametric design programs since 2010. These programs allows the precise realization of 3D projects, both individual parts and assemblies, as well as the associative generation, based on the 3D models of the 2D execution and assembly drawings. Its are called "parametric design programs" because at any time you modify a previously defined size or property (parameter), automatic whole modeled body changes. This attribute saves time and facilitates the multiple versions which can then be virtually tested. It also allows the creation of families of parts, which have the same characteristics, but different sizes; for example, you build a single pencils holder, but by editing a table containing surface dimensions, are assigned multiple values for the same size (i.e for height) and for each value assigned you get a different pencils holder. These programs are made in modular form, including modules for 3D modeling of individual parts and assemblies, 2D drawing, modeling of sheet metal parts, import/export of various formats and photorealistic representation (rendering). Programs are used extensively in industry, in the design. The resulted files can be exported in different formats (.stl, .igs, .stp or other) that can be read by machine tools with numerical control and thus can be obtained physical parts.