Galactic Sector ZZ9 Plural Z Alpha is a galactic sector containing Earth, Barnard's Star, and probably other nearby systems. The "Plural" designation may indicate probability problems; after Fenchurch disappeared during a hyperspace jump, it was pointed out to Arthur Dent that the small print advised against hyperspace travel for those born in Plural sectors. This is apparently common knowledge throughout the rest of the galaxy.
Behind the scenes
It also gives its name to the Official Hitchhiker's Appreciation Society.