Specific to the US Postal Service is the PostNet (Postal Numeric Encoding Technique) barcode. Examples can be found on mail in the United States. It is unique in that it varies the height instead of the width of the bars. Except for the start and end bars, they are in groups of five, with two long and three short. A PostNet barcode can encode five, nine, or eleven digits, nothing else. It includes a checksum.