Code 39 (also called “3 of 9 Code”) was developed by Dr. David Allais and Ray Stevens in 1974. Code 39 is a discrete, variable length symbology and was the first alphanumeric bar code to be developed. It is designed to encode twenty-six uppercase letters (A-Z), numeric digits (0-9), and seven special characters: space, minus (-), plus (+), period(.), dollar sign ($), slash(/), and percent (%). It can be extended to encode the ASCII character set by using a two character coding scheme. Each character is composed of nine elements: five bars and four spaces. Three of the elements are wide (binary value 1) and six elements are narrow (binary value 0), the width ratio can be chosen between 1:2 and 1:3.
Code 39 is one of the only type of barcodes in common use that does not require a checksum. An inadequately interpreted bar cannot generate another valid character. The template must then add a fixed asterisk (*) before and after the data and print the field using a Code 39 barcode font. This “self-checking” makes it especially attractive for applications where it is inconvenient to perform calculations each time a barcode is printed.