Barcode Generator

Why check digits exist

A check digit is a small amount of redundancy that catches the mistakes humans and scanners actually make. The EAN scheme — alternating weights of one and three — is chosen deliberately: it detects every single-digit error, and it detects most transpositions of adjacent digits, which is the classic typing mistake. It is not a security feature and cannot detect every possible error; two compensating mistakes can still produce a valid-looking number. What it does do is make it very unlikely that a mis-scanned or mistyped code silently resolves to a different real product, which is exactly the failure mode a retail system cannot tolerate.

How a linear barcode is read

A scanner does not see digits; it sees a one-dimensional signal of light and dark as its beam or sensor crosses the symbol, and it measures the relative widths of the bars and spaces. That is why the printed module width matters so much and why stretching one axis destroys a code: the decoder is comparing widths against each other, and distorting them breaks the comparison. It is also why EAN encodes the left half of the code with two different digit tables selected by parity — the parity pattern encodes the thirteenth digit, which has no bars of its own, and it lets the scanner work out which way round the symbol is being read. A code scanned upside down decodes correctly because the parity tells the decoder to reverse it.