International Standard Musical Work Code
Also known as ISWC
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Short answer
An International Standard Musical Work Code (ISWC) is the ISO 15707 identifier for a musical work: the composition, meaning its melody and any lyrics, independent of any recording. It is written as the letter T, nine digits and a check digit, for example T-123.456.789-C, and is usually assigned by a performing rights organization when the work is registered.
A work, not a recording
Music copyright distinguishes two things: the musical work written by songwriters and controlled by publishers, and the sound recording of a performance of it, the master. The ISWC identifies the first; the ISRC identifies the second. A song covered by ten artists has one ISWC and at least ten ISRCs.
Format
An ISWC is the prefix letter T followed by a nine-digit work identifier and a single check digit, conventionally displayed as T-123.456.789-C. The dots and hyphens are for display only. The code carries no meaning beyond identity: it does not encode a country, a publisher or a year.[1]
How an ISWC is assigned
The ISWC system is coordinated by CISAC, the International Confederation of Societies of Authors and Composers, through local agencies — in most countries the performing rights organizations.[1][2] A code is usually issued when a work is first registered with a society, together with its writers, their shares and their interested-party identifiers. A work registered twice with conflicting writer shares is a common reason for a delayed ISWC, and for delayed royalties.
Questions
Do I need an ISWC to release a song?
No store requires one to accept a release, but publishing royalties are matched more reliably when the work has one, so registering the composition with a performing rights organization soon after release is good practice.
What is the difference between an ISWC and an ISRC?
An ISWC identifies a composition; an ISRC identifies a specific recording of it. One ISWC can be linked to many ISRCs.
Who gives out ISWCs?
Local ISWC agencies coordinated by CISAC — in practice usually the songwriter's performing rights organization, when the work is registered.