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).


Source: The Jargon File