Also called "Code 3 of 9" or "Alpha39," the Code 39 barcode was the first code to use both numbers and letters. It is a variable length barcode that can encode up to 43 alphanumeric characters at one time. It is most commonly used in the military and the automotive industry.
Specifications: The Code 39 barcode has a start and end symbol to define the beginning and end of the code for the scanner, commonly represented as a * in regular fonts. Aside from the start/end character, the barcode can technically only encode the numbers 1-10. But using special designations, it is able to designate letters by dividing them up into categories. For example, the first 10 letters (A-J) are given numeric values, preceded by a "Letters" designation. The next 10 (K-T) are designated "Letters +10." So K would be Letters +10, followed by a 1, showing that it's the 11th letter of the alphabet. U-Z then become "Letters +20." And numbers, of course, have their own designation as well.
The use of both letters and numbers makes Code 39 more versatile. In addition, it is self-checking, so it does not require a check number (though one is still recommended).
It is limited to a maximum of 43 characters. Also, its method of assigning number values to letters in order to read them limits versatility and precludes other characters.