The UPC-A barcode is an optical pattern of bars and spaces that format and encode the UPC digit string. Each digit is represented by a unique pattern of two bars and two spaces. The bars and spaces are variable width; they may be one, two, three, or four modules wide. The total width for a digit is always seven modules. To represent the 12 digits of the UPC-A code requires a total of 7×12 = 84 modules.
A complete UPC-A includes 95 modules: the 84 modules for the digits (L and R) combined with 11 modules for the start, middle, and end (S, M, and E) patterns. The S and E patterns are three modules wide and use the pattern bar-space-bar; each bar and space is one module wide. The M pattern is five modules wide and uses the pattern space-bar-space-bar-space; each bar and space is one module wide. In addition, a UPC symbol requires a quiet zone (additional space modules) before the S pattern and another quiet zone after the E pattern.