The most famous computer that never was. PDP-10 computers

running the TOPS-10 operating system were labelled

"DECsystem-10" as a way of differentiating them from the

PDP-11. Later on, those systems running TOPS-20 were

labelled "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 colour 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).