Code 39

Code 39 (or Code 3 of 9) differs from most of the other popular bar code symbologies in that it is alphanumeric. Although it originally could encode only 39 different characters, it can now encode 43;

01234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ.,%$-/ and the space character.

There is also a start and stop character, commonly represented as an asterisk (*).
The asterisks are encoded into the barcode, but their representation in the human readable text is optional.

In industrial bar coding systems, it is Code 39 that is most frequently used, although it is more widespread in the U.S. than Europe and elsewhere.

Code 39 employs two different widths of bar, termed narrow and wide. The wide bar can be between 2 and 3 times the width of the narrow bar. This is known as the wide/narrow ratio. The narrow bar may be between 0.0075" and 0.05". This means that a wide range of sizes and printed tolerances are possible, although Code39 is not as compact as other bar code types.

The code height is not dependent on narrow bar or ratio and tends, therefore, to be defined by individual applications.

The internal security of the Code 39 bar code means that misreads are rare, but in order to guard against incorrect data entry at the point of creating the bar code, a check character is sometimes used.

Many non-retail bar code applications specify Code 39, although some have recently switched to use Code 128.

In addition, many in-house bar code systems use Code39 as there is no theoretical limit on code length. Also, the flexibility of ratio and narrow bar width means that size (and therefore printed tolerance) can be adjusted to suit the substrate and method of printing.