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).