Read-Backward Polyphase Sorting
Read-backward Polyphase sorting provides more
efficient use of the tapes available to a sort 
than most other sorting techniques.  Backward Polyphase
produces a continuous merging process from n-1 
tapes where n is the total number of tapes being used in
the sorting process.  Any of the available presorting 
techniques may be used in conjunction with the Polyphase
merge sort provided that the presort has the 
capability of producing both ascending and descending
strings and distributing the strings on the various 
tapes as required by the Polyphase Merge.
CACM May, 1963
Gilstad, R. L.
