The UPC-A's left-hand side digits ( the digits to the left of the M ( middle ) guard pattern ) have odd parity, which means the total width of the black bars is an odd number of modules, on the contrary, the right-hand side digits have even parity.
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After seeing a S ( start ) or E ( end ) guard pattern ( they are the same, " bar-space-bar ", whichever direction they are read ), the scanner will first see odd parity digits, if scanning left-to-right, or even parity digits, if scanning right-to-left . With the parity / direction information, an upside-down symbol will not confuse the scanner.
