Before the DSR was developed, I thought that the only proper way to analyze seismic data was to decompose it into Snell waves. Since a Fresnel zone seems to be about 10 wide, not many Snell waves should be required. The small number of required sections was important because of the limited power of computers in the 1970's. I knew that each Snell wave is analyzable by a single square-root equation, and that even multiple reflections can be handled by methods described in FGDP and in this book. Theoretically this approach was a big improvement over CDP stack, which is hardly analyzable at all. A practical problem for downgoing Snell waves, however, is that they may become complicated early if they encounter lateral velocity inhomogeneity shortly after they depart the earth's surface. I no longer believe that Snell waves are a panacea, although I am unsure what their ultimate role will be. But many waves behave a little like they are Snell waves. This motivates the development of a coordinate system that is ideal for Snell waves, and good for waves that are not far from being Snell waves.