The model consists of an "infinite series" of the elements shown in the figure, and the values of the components are specified "per unit length" so the picture of the component can be misleading. formula_3, formula_4, formula_5, and formula_6 may also be functions of frequency. An alternative notation is to use formula_11, formula_12, formula_13 and formula_14 to emphasize that the values are derivatives with respect to length. These quantities can also be known as the primary line constants to distinguish from the secondary line constants derived from them, these being the propagation constant, attenuation constant and phase constant.