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Those are mail-sorting barcodes. In the US, the long series of tall and short bars along the bottom of the envelope is the Intelligent Mail barcode (IMb): a sorting machine reads the handwritten or printed address, encodes the destination into that barcode, and every machine downstream reads the barcode instead of re-reading the address. They carry nothing you can't already see on the envelope.
| Barcode you might see | What it is | What it encodes |
|---|---|---|
| Long bar pattern along the bottom edge (US) | Intelligent Mail barcode (IMb) | Routing ZIP+4 code plus a mailer ID and serial number for the sender's tracking |
| Short/long bars in a single row above the address (older US mail) | POSTNET (now retired) | Only the ZIP code; replaced by IMb around 2013 |
| Barcode sprayed on the back or front (Canada) | Canada Post sort barcode | Routing data applied during automated sorting |
| Barcode pre-printed by the sender near the address | Sender-applied IMb | Same destination data, applied before the piece ever reaches a postal facility |
The first machine a letter meets does the hard work: cameras read the address, match it against the postal database, and spray or print the barcode. After that, high-speed sorters just read the barcode, which is faster and far more accurate than re-reading handwriting thousands of times. That is why a clearly printed address in block letters sails through while ornate cursive gets kicked out for a human to look at.
Not beyond what's already on the envelope. The routing portion encodes the destination ZIP or postal code, which anyone holding the envelope can read in plain text. The tracking portion encodes the sender's own ID and a serial number so the sender can see when the piece passes a checkpoint. No barcode on ordinary mail contains your name, your account numbers, or anything hidden: the worry people have about "secret codes" comes from not knowing they're just machine-readable versions of the address.
Because the barcode is a memory of the last routing decision. If you write "not at this address" on an envelope but leave the barcode intact, the sorting machine reads the barcode first, ignores your handwriting, and routes the piece right back to your mailbox. One thick line through the barcode forces the machine to read the address fresh and make a new decision. This is the single most effective trick for stubborn misdelivered mail.
You shouldn't. Mailers who pre-print their own IMb (banks, utilities, big senders) do it because it earns them discounted postage and gives them delivery tracking. If you try to print your own barcode on an envelope without knowing the encoding, a bad barcode is worse than none: the machine trusts it, routes wrong, and the piece ends up on a detour. Ordinary senders should just leave the barcode area alone and write the address clearly.
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