Question 1

What is a check digit?

Accepted Answer

A check digit is a calculated digit appended to a barcode to verify its accuracy during scanning. It is derived from the other digits using a specific algorithm.

Question 2

How does the EAN/UPC check digit work?

Accepted Answer

The algorithm multiplies each digit by alternating weights of 1 and 3, sums the products, and calculates the number that rounds the total up to the nearest multiple of 10.

Question 3

What is the ISBN-10 check digit?

Accepted Answer

ISBN-10 uses a modulo-11 algorithm with weights decreasing from 10 to 1. The check digit can be 0-9 or X (representing 10).

Question 4

Can a check digit always detect errors?

Accepted Answer

Check digits catch single-digit errors (e.g., 5 → 6) and most transposition errors (e.g., 12 → 21). No error-detection scheme catches all errors, but check digits reduce scanning errors significantly.

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