An ISIL is alphanumeric, with a maximum of 16 characters. Valid symbols are A-Z, 0-9, solidus, hyphen-minus and colon.
An ISIL consists of a prefix identifying the authority which issued the ISIL, a dash, and then an identifier issued by that authority. All two letter prefixes are reserved for the ISO 3166-1 alpha-2 country code, followed by an identifier assigned by that country's national library authority. Global-level identifiers can also be assigned, which are not associated with a particular country, e.g. 'oclc-' for the OCLC. The suffix is generally a pre-existing system of identifying libraries; thus, ISIL unifies existing systems around the world rather than instituting an entire system from scratch.