An International Securities Identification Number (ISIN) is an alphanumeric code that uniquely identifies a security.
An ISIN consists of a two character country code, a nine digit number and one check digit.
Each country where ISIN numbers are used has a single national numbering agency to assign numbers using that country code. This guarantees that the same ISIN cannot be issued to two different securities.
ISINs are not the only codes used to identify securities. They are not even always the most useful. One problem is that nine digit numbers are not very memorable. For this reason, most exchanges have other coding systems that are easier for human beings to use (to call up quotes or enter trades), while the computers that handle clearing and settlement are more likely to use ISIN codes.
It can be seen from this example that Reuters and Bloomberg codes are based on the TIDM together with an extension to identify the London market, and that the ISIN for UK companies is based on the SEDOL together with a country code (GB) and a check digit (the 6 at the end in the example above).