International Standard Musical Work Code
An International Standard Musical Work Code (ISWC) is used to uniquely identify a musical composition. It is an official standard (ISO 15707).
An ISWC consists of the letter T followed by nine digits (in three groups of three) and then a check digit, like
The first ISWC ever issued was for the song "Dancing Queen" by ABBA, which got ISWC
These codes are for works (such as songs and symphonies); specific recordings of them should have International Standard Recording Codes.