Each character in the barcode symbol is composed of three bars and three spaces. (The stop adds an additional extra bar of length 2.) Each bar or space is 1, 2, 3 or 4 units wide, the sum of the widths of bars must be even, the sum of the widths of the spaces must be odd, and total 11 units per character. For instance, encoding the ASCII value 0 can be viewed as 10011101100, where a 1(One) is a bar and a 0 is a space. A combination which contains a single 1 would be the thinnest line in the bar code. A combination including three 1 (111) in sequence indicates a bar three times as thick as a single 1 bar.
The code uses 107 of the 108 possible patterns that fulfill the required criteria, with the pattern 211133 deliberately remaining unused. This is done to enhance omnidirectional scanning and help prevent misreads (if 211133 represented a legitimate symbol it could be mistaken for the stop code 2331112 when scanned backwards.
Code 128 Set a uses ASCII characters 00 to 95 (0-9, A-Z and control codes), special characters, and FNC 1-4. Code 128 has three types of sets i.e. Code Set A, Code Set B and Code set C.