The use of computers to aid in the identification of phases from their powder diffraction patterns was pioneered in the mid 1960's by Frevel, Nichols and Johnson (1-3). Today's most widely used Johnson algorithm conducts a reverse sequential search by comparing each reference pattern in the JCPDS powder diffraction file (PDF) to the unknown pattern. A figure of merit is computed for each match and the patterns with the best figures of merit are listed at the end of the search. The Nichols approach is a reverse search of a singly inverted reference file. An inverted file is one which stores the reference patterns according to the d value of the lowest angle 100% intensity line (d1) . This type of file is analogous to the Hanawalt search books distributed by the JCPDS for manual searching. When an inverted file is stored in a random format, along with suitable disk directory files, only reference patterns containing d1 values of interest need be read in the search.