What is the barcode that the Post Office uses on letters?
The barcode that you commonly find on your mail is called Postnet. It looks like the code below. It is used by the Post Office to encode Zip Code information for automatic mail sortation. Postnet differs drastically from normal barcodes in that information is encoded by having bars of two different heights rather than different widths. For this reason postnet code cannot be read by conventional barcode readers.
When the Post Office created this code their objective was to have a code that could be easily printed using a variety of printing techniques. It is not necessary to have a very high resolution printer to print postnet. Older dot matrix printers will do an adequate job. One important factor to remember about postnet is that the code symbol is not scaleable. In other words, the size of the symbol can not change. The bar width and space width as well as the height of the symbol is always the same. Only the length of the symbol will change. A symbol that encodes a regular five digit Zip code will be shorter than one for a Zip + 4 code.
Many barcode printing programs, such as B-Coder by TAL can be used to create postnet symbols. If you want to set up a program on your PC to include the code on mailing labels to reduce postage cost, you might want to look into this or other programs that will print postnet code.