Editing Code 128 (section)

===Start/stop and bar widths===
Each symbol in the barcode is composed of three bars and three spaces. Each bar or space is 1, 2, 3 or 4 units wide, the sum of the widths of bars must be even (4, 6 or 8 units), the sum of the widths of the spaces must be odd (3, 5 or 7 units), and total 11 units per symbol. For instance, encoding the ASCII character "0" can be viewed as 10011101100, where a sequence of 1's is a bar and a sequence of 0's is a space. A single 1 would be the thinnest line in the bar code. Three 1's in sequence (111) indicates a bar three times as thick as a single 1 bar.

There are 108 possible 11-unit wide symbols, and the code uses all possible symbols. Two of the symbols are used for stop (end-of-barcode) indication, stop and reverse stop. The two stop symbols are special because they are always followed by a 2-unit bar, forming a 13-unit long stop pattern. Reading the stop pattern left to right is the stop symbol (followed by a 2-unit bar), and reading the stop pattern right to left is the reverse stop symbol (followed by a 2-unit bar).