PDP-20: n. The most famous computer that never was.

: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).
-- The AI Hackers Dictionary