The "Z" in "Z1" comes from "Zukunft" -- the German word for "future". It is the Z1 that has set the standard for BMW naming its current roadsters "Z-something", i.e. the Z3 and Z8. I believe there was a Z12 or Z13 concept car (small, closed car), and of course the Z9 concept car BMW showed in Frankfurt, and later also in New York...