Editing Universal Product Code (section)

== Encoding ==
{|class="wikitable floatright collapsible" style="text-align:center;font-size:85%;"
|+UPC number encoding
! Value
! colspan=7 | L encoding
! rowspan=11 style="font-size:10%; | &nbsp;
! colspan=7 | R encoding
|-
! 0<!--3211-->
| style="background:#fff;color:#000;" colspan=3 | 3
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=3 | 3
| style="background:#fff;color:#000;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=1 | 1
|-
! 1<!--2221-->
| style="background:#fff;color:#000;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#fff;color:#000;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#fff;color:#000;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#fff;color:#000;" colspan=1 | 1
|-
! 2<!--2122-->
| style="background:#fff;color:#000;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#fff;color:#000;" colspan=2 | 2
|-
! 3<!--1411-->
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=4 | 4
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=4 | 4
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=1 | 1
|-
! 4<!--1132-->
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=3 | 3
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=3 | 3
| style="background:#fff;color:#000;" colspan=2 | 2
|-
! 5<!--1231-->
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#fff;color:#000;" colspan=3 | 3
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=3 | 3
| style="background:#fff;color:#000;" colspan=1 | 1
|-
! 6<!--1114-->
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=4 | 4
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=4 | 4
|-
! 7<!--1312-->
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=3 | 3
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=3 | 3
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=2 | 2
|-
! 8<!--1213-->
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=3 | 3
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=3 | 3
|-
! 9<!--3112-->
| style="background:#fff;color:#000;" colspan=3 | 3
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=2 | 2
| style="background:#000;color:#fff;" colspan=3 | 3
| style="background:#fff;color:#000;" colspan=1 | 1
| style="background:#000;color:#fff;" colspan=1 | 1
| style="background:#fff;color:#000;" colspan=2 | 2
|}

The UPC-A barcode is visually represented by strips of bars and spaces that encode the UPC-A 12-digit number. Each digit is represented by a unique pattern of 2 bars and 2 spaces. The bars and spaces are variable width, i.e. 1, 2, 3, or 4 modules wide. The total width for a digit is always 7 modules; consequently, UPC-A 12-digit number requires a total of {{nowrap|7&times;12 {{=}} 84 modules}}.

A complete UPC-A is 95 modules wide: 84 modules for the digits (L and R sections) combined with 11 modules for the S (start), M (middle), and E (end) guard patterns. The S (start) and E (end) guard patterns are 3 modules wide and use the pattern ''bar-space-bar'', where each bar and space is one module wide. The M (middle) guard pattern is 5 modules wide and uses the pattern ''space-bar-space-bar-space'', where each bar and space is also one module wide. In addition, a UPC-A symbol requires a quiet zone (extra space of 9 modules wide) before the S (start) and after the E (end) guard patterns.

{| class="wikitable" style="text-align:center; background-color:#80A080"
|+ Encoding table for UPC-A barcode pattern S<u>L</u>LLLLLMRRRRR<u>R</u>E
! rowspan="2" | Quiet<br />zone
! rowspan="2" | S<br />(start)
! colspan="10" | L<br />(left numerical digit)
! rowspan="2" | M<br />(middle)
! colspan="10" | R<br />(right numerical digit)
! rowspan="2" | E<br />(end)
! rowspan="2" | Quiet<br />zone
|-
! 0 !! 1 !! 2 !! 3 !! 4 !! 5 !! 6 !! 7 !! 8 !! 9 !! 0 !! 1 !! 2 !! 3 !! 4 !! 5 !! 6 !! 7 !! 8 !! 9
|- valign="top"
| [[File:UPC-A Q.svg|27x250px]] || [[File:UPC-A S.svg|10x250px]] || [[File:UPC-A L0.svg|21x235px]] || [[File:UPC-A L1.svg|21x235px]] || [[File:UPC-A L2.svg|21x235px]] || [[File:UPC-A L3.svg|21x235px]] || [[File:UPC-A L4.svg|21x235px]] || [[File:UPC-A L5.svg|21x235px]] || [[File:UPC-A L6.svg|21x235px]] || [[File:UPC-A L7.svg|21x235px]] || [[File:UPC-A L8.svg|21x235px]] || [[File:UPC-A L9.svg|21x235px]] || [[File:UPC-A M.svg|15x250px]] || [[File:UPC-A R0.svg|21x235px]] || [[File:UPC-A R1.svg|21x235px]] || [[File:UPC-A R2.svg|21x235px]] || [[File:UPC-A R3.svg|21x235px]] || [[File:UPC-A R4.svg|21x235px]] || [[File:UPC-A R5.svg|21x235px]] || [[File:UPC-A R6.svg|21x235px]] || [[File:UPC-A R7.svg|21x235px]] || [[File:UPC-A R8.svg|21x235px]] || [[File:UPC-A R9.svg|21x235px]] || [[File:UPC-A S.svg|10x250px]] || [[File:UPC-A Q.svg|27x250px]]
|}

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. Consequently, a UPC scanner can determine whether it is scanning a symbol from left-to-right or from right-to-left (the symbol is upside-down). 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. When confronted with an upside-down symbol, the scanner may simply ignore it (many scanners alternate left-to-right and right-to-left scans, so they will read the symbol on a subsequent pass) or recognize the digits and put them in the right order. There is another property in the digit encoding. The right-hand side digits are the optical inverse of the left-hand side digits, i.e. black bars are turned into white spaces and vice versa. For example, the left-hand side "4" is {{nowrap|space×1 - bar×1 - space×3 - bar×2}}, meanwhile the right-hand side "4" is {{nowrap|bar×1 - space×1 - bar×3 - space×2}}.