The classical physicist, being concerned largely with isolated transmission systems, has emphasized only one wave concept, that of the velocity of propagation or more generally of the propagation constant. But the communication engineer who is interested in "chains" of such systems from the very start is forced to adopt a more general attitude and introduce the second important wave concept, that of the impedance. The physicist concentrates his attention on one particular wave: a wave of force, or a wave of velocity or a wave of displacement. His original differential equations may be of the first order and may involve both force and velocity; but by tradition he eliminates one of these variables, obtains a second order differential equation in the other and calls it the "wave equation." Thus he loses sight of the interdependence of force and velocity waves and he does not stress the difference which may exist between waves in different media even though the velocity of wave propagation is the same. The engineer, on the other hand, thinks in terms of the original "pair of wave equations" and keeps constantly in mind this interdependence between force and velocity waves.