1. A computer-implemented method for designing a CAD modeled object, wherein the method comprises: displaying several three-dimensional parametric curves on a screen modeled by a function C(u)=(x(u), y(u), z(u)), wherein each of the several three-dimensional parametric curves is divided in several portions according to pointing points of a curve given by P u =(x(u), y(u), z(u)) to a different design operation based on a value of u, wherein each three-dimensional parametric curve comprises a central portion, to which a moving operation is assigned and at least one extremity portion, to which an extension operation is assigned, for a given curve, there being a different design operation being assigned to each portion of the curve; user-interacting with the screen; determining one portion upon a location of the user-interacting on a support, the determining being performed among all the portions; and performing, on the curve to which the determined portion belongs, the operation assigned to this portion.