In order to analyze the geometric quality of any surface we have defined a shape language that can be used in tolerancing and metrology softwares. Modal parameters defines a shape langage allowing to describe geometric variations associating undulation, form, position, orientation and dimensions. It defines a geometric basis, easy to use by a simple user or deeply by an expert. The principal properties of this basis are the exhaustiveness and the metric of the parameters. We can use either natural mode shapes that can be modified by technological mode shapes.