Source : Free On-Line Dictionary of Computing

PDP-20
     
        The most famous computer that never was.  {PDP-10} computers
        running the {TOPS-10} operating system were labelled
        "DECsystem-10" as a way of differentiating them from the
        {PDP-11}.  Later on, those systems running {TOPS-20} were
        labelled "DECSYSTEM-20" (the block capitals being the result
        of a lawsuit brought against DEC by Singer, which once made a
        computer called "system-10"), but contrary to popular lore
        there was never a "PDP-20"; the only difference between a 10
        and a 20 was the {operating system} and the colour of the
        paint.  Most (but not all) machines sold to run {TOPS-10} were
        painted "Basil Blue", whereas most TOPS-20 machines were
        painted "Chinese Red" (often mistakenly called orange).
     
        [{Jargon File}]
     
        (1994-12-21)