The ISIL is a variable length identifier. The ISIL consists of a maximum of 16 characters, using digits (Arabic numerals 0 to 9), unmodified letters from the basic Latin alphabet and the special marks solidus (/), hyphen-minus (-) and colon (:). Latin letters modified with one or more diacritics and letters from alphabets other than Latin cannot be used in the ISIL.

Each ISIL identifier shall be unique when normalized to the repertoire of characters specified in ISO/IEC-10646-1. 

ISIL country code and non-country code prefixes are case insensitive. The case sensitivity of the library code shall be decided at the national level. For instance, in Finland FI-Ht and FI-HT are two different ISILs. In the UK, GB-UkLoRHS and GB-uklorhs are two variants of the same ISIL and identify the same organization.

An ISIL is made up by two components: a prefix and a unit identifier, in that order, separated by a hyphen-minus. The hyphen-minus is a mandatory character in the ISIL string.

Country code prefix


A country code identifies the country in which the library or related organization is located at the time the ISIL is assigned. The country code shall consist of two uppercase letters in accordance with the codes specified in ISO 3166-1. All two-character alphabetic combinations are reserved for possible future assignment as country codes, thus may not be assigned as non-country code prefixes.

EXAMPLE 1 GB = United Kingdom
EXAMPLE 2 DE = Germany
EXAMPLE 3 SE = Sweden

When the library has components located in different geographic areas, the country code is based on the location of the main library or of the main administrative unit.

Non-country code prefixes

A non-country code prefix is any combination of Latin alphabet characters (upper or lower case) or digits (but not special marks). The prefix may be one, three, or four characters in length. The prefix is registered at a global level with the ISIL Registration Authority.

EXAMPLE                     ZDB = Staatsbibliothek zu Berlin - Zeitschriftendatenbank

Unit identifier

The Unit identifier is an alphanumeric element designed to accommodate existing national systems of library identification or newly assigned identifiers for libraries or related organizations. Already existing identifiers, when used in international contexts, will necessarily require the addition of the two uppercase letters of the respective country code, followed by a hyphen-minus character. If appropriate when assigning a new identifier to a library or related organization, it is recommended that the Unit identifier of the ISIL include the element indicating the geographic subdivision (state, province, region, city, etc.) where the library or the related organization is located. If the geographic subdivision element is used, it is recommended that the element be in accordance with the codes specified in ISO 3166-2.

The Unit identifier will have up to 11 character positions.


When an ISIL is written, printed, or otherwise visually presented, it shall be preceded by the letters ISIL separated from the identifier by a space.

  Illustration:    ISIL DK-710100 

Opdateret 07. August 2020