[Kon-Peki]

Those are also called fire addresses, because they were assigned by the fire department rather than the postal service. 6 or 8 decades ago, the post office decided to accept them as official rather than force people to change.

They have their own section in the postal addressing standards, under the "Unusual Addressing Situations":

https://pe.usps.com/text/pub28/pub28apd_004.htm

[susiecambria]

Thanks for this. . . I've been writing lots of GOTV letters across the country and paying attention to the addresses, rather than simply writing them on the envelopes, help make the process interesting.

I spent 20+ years in Washington, DC so am used to NW, NE, SW, and SE with avenues, numbered streets, flower streets of two syllables, blah blah blah. But I now live in rural Virginia and state routes are the norm.

Your explanation got me to focus more on what and why. And, I learned something, so thank you.