The essence of Backus theory Backus (1962) is that it allows a bunch of layers to be replaced by a single layer. The new homogeneous medium has elastic properties identical in the long-wavelength limit so that mass and travel-time are conserved, but wavelet shape is not. In this paper I show that for normal incidence plane waves the three elastic layer parameters of thickness, compliance, and density can be replaced by the two, travel-time and impedance, without losing reflection and transmission information.