The International Standard Music Number is a "unique number for the identification of all printed music publications ... whether a part, a score, or an element in a multi-media kit" that became available at the end of 1993.
ISMN numbers from 1993 through 2007 are 9-digit numbers beginning with the letter "M"
ISMN numbers after 2007 are 13-digit numbers beginning with the 4 numbers "9790" (source)
Both may appear on scores:
The ISMN is entered in the 024 field.
|9-digit number||13-digit number
(the first 4 numbers will be 9790)
|first indicator||= 2||first indicator||= 2*|
|second indicator||= blank||second indicator||= blank|
* Previously, the 13-digit ISMNs were treated as EANs, which were coded 1st indicator value 3 source. As of June 2013, all ISMNs (both 10- and 13-digit) should be coded with a first indicator value of 2. source. When transcribing the old format ISMN, include the initial letter M, followed by the numbers without spaces and hyphens. The new format ISMNs are transcribed without spaces.
If a publication has more than one ISMN and we choose not to transcribe all of the ISMNs, follow these guidelines, originally published in Fontes Artis Musicae, vol. 40/2, p. 148:
If the publication includes a score, give the ISMN of the score. If the publication includes parts,
give the ISMN of the score, if any, and the ISMN with the lowest item number among those of
the parts (e.g., of three parts with item numbers, 12345, 12344, and 12346, choose, 12344).
When a number on a publication is given as an ISMN number, but does follow the standards of ISMN, tag it in the 024 field with indicator 2, but put the number in ‡z:
|On item:||ISMN M-69400-039-40419|
|In bib record:||024 2_ ‡z M6940003940419|