Code 39, the first alpha-numeric symbology to be developed, is still widely used in non-retail industry. It is the standard bar code used by the United States Department of Defense, and is also used by the Health Industry Bar Code Council (HIBCC). Code 39 is also known as "3 of 9 Code" and "USD-3".
The symbol can be as long as necessary to store the encoded data. It is designed to encode 26 uppercase letters, 10 digits and 7 special characters. It can be extended to code all 128 ASCII characters by using a two character coding scheme.
Each data character encoded in a Code 39 symbol is made up of 5 bars and 4 spaces for a total of 9 elements. Each bar or space is either "wide" or "narrow" and 3 out of the 9 elements are always wide. That's what gave the code its other name - Code 3 of 9.
Code 39 does not require a checksum, although a modulo 43 check digit may be appended for increased data integrity (the Mod 43 checksum is seldom used). Code 39 is just about the only type of barcode in common use that does not require a checksum. This makes it especially attractive for applications where it is inconvenient, difficult, or impossible to perform calculations each time a barcode is printed. For example, when performing a word processor merge operation there is generally no easy way to calculated a checksum if one of the merge data fields is to be barcoded. With Code 39, however, no checksum is needed; the merge template document must simply add a fixed asterisk (*) before and after the data and print the field using a Code 39 barcode font.