The time-temperature superposition principle, frequently used to describe the mechanical and electrical relaxation behaviour of polymers, is considered within the more general concept of scaling with translation path parallel to the abscissa. The most general functions leading to scaling with translation path parallel to the horizontal axis are given. Furthermore, the meaning of the matching of the different curve segments, when the composite or master curve is constructed, is discussed. Finally, the concepts developed are applied to particular functions used for polymers and to the Williams- Landel-Ferry equation.