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The International Music Score Library Project (IMSLP), also known as the Petrucci Music Library after publisher Ottaviano Petrucci, is a project for the creation of a virtual library of public domain music scores, based on the wiki principle.
The Choral Public Domain Library (CPDL) is a sheet music archive which focuses on choral and vocal music in the public domain or otherwise freely available for printing and performing (such as via permission from the copyright holder).
Classical Music from Alexander Street supports the teaching and research of classical music with coverage of all major genres and time periods from medieval to contemporary, from choral works to symphonies, operas and the avant-garde.
Sibley Music Library is the library of the Eastman School of Music, Rochester, NY. It was founded in 1904 by Hiram Watson Sibley in honor of his father Hiram Sibley and is currently the largest university music library in the US.