Here in an Appendix are some paraphrases with updated annotations are presented of the Author's changing ideas over the years about relative permeability phenomena. These have been contained in a series of publications from 1948 to date, however not always in a concordant way. In point of fact, some initially were chosen to be consistent merely with empirical information. More recently, however, the papers have tended to be based on the prescripts of non-equilibrium thermodynamics.
In one way or another, and indeed now for almost a Century, the voluminous literature on the subject has substantially been influenced by the evolving understandings about the meanings and applicability of what today still remains to be a perplexing (rather than an ubiquitous) relative permeability concept. Some might say (but others, not) that this very subject already has been sufficiently dealt with by generations of previous authorities. Outstanding ones to be mentioned here are Richards (1931), Leverett (1942), and Yuster (1951), but passing reference also will be made to the remarkable achievements of other gigantic innovators. Any mention a few of them would surely include the works of Edgar Buckingham (1907), Josef Kozeny (1927), M. King Hubbert (1935 and 1956), P. C. Carman (1937), G. L. Hassler (1944), Morris Muskat (1937 and 1949), J. R. Philip (1955) to date, A. E. Scheidegger (1957 as revised in 1974), M. R. J. Wyllie (1962), E. C. Childs (1969), Jacob Bear (1972) to date, Francois Kalaydjian in the 1980s and 1990s, and Steve Whitaker et al (1986 et.seq.). Even so, the simple aim held by the Author of the present paper is to show that further clarifications still are urgently needed. And the focus of major importance now should have to do with how best to measure and to apply relative permeability data whenever simulation studies of reservoir transport processes are to be undertaken.