PDP-20

PDP-20 n.  The most famous computer that never was.
   {PDP-10} computers running the {{TOPS-10}} operating system
   were labeled `DECsystem-10' as a way of differentiating them from
   the PDP-11.  Later on, those systems running {TOPS-20} were labeled
   `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 color 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).