text: what kind of system was used in determining and assigning zip codes to each area of the us? i'm just curious because obviously they didn't just say ok this are is 1 and this one is 2 and so on.. so how did they derrive the numbers for each area? was there some kind of system? it's geographical from east to west.  you'll notice that zip codes on the east coast are lower numbers than zip codes on the east coast.  as always, wikipedia is the source for all knowledge.  check out the source i cite.