What have we done? Firstly we have shown that Backus exactly translates from depth and elastic properties to time and impedance. In other words we have transplanted Backus from the physical world to the data processor's world of traveltime and reflectivity as characterized by Goupillaud's ladder and Chapter 8 of FGDP Claerbout (1976). Secondly we have swapped two elastic parameters, compliance and density, for one, impedance, without any loss of information. This may represent a key to extending Backus theory sufficiently away from zero frequency to provide useful approximations to changes in wavelet shape due to scattering.