The document "International Standard ISO 6166" describes the standard used to create a uniform structure for International Securities Identification Numbers (ISINs).

An ISIN uniquely identifies a security. Its structure is defined in ISO 6166. The ISIN code is a 12-character alpha-numeric code that does not contain information characterizing financial instruments but serves for uniform identification of a security at trading and cll. An ISIN consists of three parts: generally, a two-letter country code, a nine-character alpha-numeric national security identifier, and a single check digit.

Please click   pdf ISO 6166 - check digit formula (53 KB)  to open the "International Standard ISO 6166" document.