GTIN (Global Trade Item Number)

Anatomy

A GTIN encodes a GS1 prefix (identifying the issuing country organization), a company prefix, an item reference, and a check digit computed by alternating ×3/×1 weights from the rightmost data digit. The 14-digit GTIN-14 is the superset — shorter variants are just zero-padded into it, which is why UPC↔GTIN and EAN↔GTIN conversion is trivial.

ISBN is a special case: ISBN-13 is an EAN with a Bookland prefix (978/979), so book validation is GTIN validation plus a prefix check.

What the check digit proves — and does not

The mod-10 digit catches single-digit typos and most adjacent transpositions — the classic hand-entry errors. It does not prove the number is registered or that the brand owns it: marketplaces like Amazon cross-check the GS1 registry and brand ownership, so a checksum-valid but unregistered code still gets rejected.