A barcode of vertical bars

Asked for with "type": "barcode"

The bars are filled rectangles written straight into the page content stream — no image, no font, no reading software plug-in. The bars and the quiet zones fill the rectangle's width. A module is the thinnest bar of the code, the unit every other width is counted in. On a retail code the guard bars — the pair at each end and the pair down the middle — take the rectangle's whole height, and every other bar stops five modules above its foot, which is the room the figures set under the code stand in; where five modules would be over half the height, the guards take half. A Code 128 has no guard bars, so all its bars take the whole height. The symbologies a request names: Code 128 carries the printable ASCII characters. Code set C writes two digits in one symbol — an odd count opens in code set B for the first digit — and code set B writes one character per symbol, so fourteen digits take 112 modules in code set C against 189 in code set B. Left to itself a Code 128 takes code set C for a content of digits alone and code set B for anything else, and code_set names one of the two instead; a content that is not digits alone is refused in code set C. Code 39 carries the ten digits, the capital letters, the space and the six characters -, ., $, /, + and %; a lower-case letter is refused, and check_digit adds the modulo 43 check character, which a scanner hands back with the text. Code 93 carries the same characters in about half the room and always carries its two check characters. ITF-14 carries fourteen digits: thirteen are the case number and the check digit is computed, fourteen are the whole code and the last is verified. Codabar carries the ten digits and the six characters -, $, :, /, . and +, and opens and closes with one of A, B, C or D, which a scanner hands back with the rest. For EAN-13, twelve digits are the number and the check digit is computed, thirteen are the whole code and the check digit is verified; the same rule holds for UPC-A at eleven or twelve digits, for EAN-8 at seven or eight, and for UPC-E at seven or eight, whose first digit is 0 or 1. The GS1 DataBar Omnidirectional, the code of a loose retail item, carries the fourteen-digit trade item number an ITF-14 carries: thirteen digits are the number and the check digit is computed, fourteen are the whole code and the last is verified, and any other count of digits is refused. EAN-2 takes exactly two digits and EAN-5 exactly five. Text the chosen symbology can encode is accepted; anything else answers 400. An add-on is no code of its own to a scanner: it is read as the tail of the retail code beside it, and only while the two stand close. The rectangle is divided by the code's module count plus twenty — ten blank modules kept at either end — so the nine blank modules the standard asks for between the bars of the two codes mean the add-on's rectangle begins eleven modules before the retail code's rectangle ends, both drawn at the same module. Set it further off, or shorter than the code beside it, and a scanner reads the retail code alone and drops the add-on without a word: we have a badly placed add-on refused on the next version; until then, the customer who puts both on the page keeps the gap. symbology reads the names above and nothing else.