...because 6 uppercase alphanumeric characters will fit into a 36-bit word on an IBM 7090 mainframe.
And on the DECsystem-10 systems where the encoding was called SIXBIT. This explains why DEC-10 systems had 6 characters in the filename (encoded via SIXBIT into one 36-bit word) and a three character extension (encoded into one 18-bit halfword; the other had had flags or protection or something).
Google found this (http://nemesis.lonestar.org/reference/telecom/codes/sixbit.html).
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And on the DECsystem-10 systems where the encoding was called SIXBIT. This explains why DEC-10 systems had 6 characters in the filename (encoded via SIXBIT into one 36-bit word) and a three character extension (encoded into one 18-bit halfword; the other had had flags or protection or something).
Google found this (http://nemesis.lonestar.org/reference/telecom/codes/sixbit.html).