Except when it doesn't. 60447 is in three counties in Illinois. Which causes confusion when laws/regulations are applied at a county level.

yeah! There are like 12 three-county zip codes, some are really fun, like places where a boat delivers the mail and goes to multiple states along the lake. And some zips don't refer to geographical areas at all, and others are military bases.

It was still the best UI option despite that - if you entered a ZIP that corresponded to multiple states/counties we'd pop up a second box that asked you which you lived in, but for 99% of people it was all we needed