Sometime before 2010, mail arrived with a barcode on the envelope that was clearly the destination's zip code.  This was the POSTNET barcode (POSTal Numeric Encoding Technique).   Wikipedia covers it nicely here .   When a longer looking barcode replaced the original, I wondered what it was all about.  It is the Intelligent Mail barcode (IMb).  Wikipedia has an overview article here .  Then, finding the USPS has an online decoder, well, who could resist?