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).