Isothermal melt spinning is a prototypical example of many polymer processes: The polymer melt is first subjected to simple shear flow in the extrusion die, which is followed by uniaxial extension under constant force in the spinline. In a Rheotens test, the tensile force needed for elongation of an extruded filament is measured as a function of the draw ratio. In this way, a whole set of melt spinning conditions is probed during one test run. The analysis of this complicated deformation history is simplified by the existence of Rheotens grandmastercurves. These allow a direct and straightforward description of material behavior in the fiber spinning process, and a simplified determination of an apparent elongational viscosity under the action of a constant drawdown force.