POSTNET

[1] Most often, the delivery point is added, usually being the last two digits of the address or PO box number.

The barcode starts and ends with a full bar (often called a guard rail or frame bar and represented as the letter "S" in one version of the USPS TrueType Font) and has a check digit after the ZIP, ZIP+4, or delivery point.

The following table shows the encoding for decimal digits: The ZIP+4 of 55555-1237 yields a check digit of 2 for encoded data of 5555512372 Together with the initials and terminal frame bars, this would be represented as:

There have been four formats of Postnet barcodes used by the Postal Service: A 5 digit (plus check digit) barcode, containing the basic ZIP Code only, referred to as the "A" code.

This is usually referred to as the DPBC, or Delivery Point Bar Code.

The Intelligent Mail barcode was originally supposed to be required beginning May 2011 however the USPS postponed the requirement date, allowing mailers to continue receiving automation discount rates using the POSTNET barcode until January 28, 2013, at which time Intelligent Mail barcode was required for those reduced rates.