Which work is known as a 20-volume reference for Western music that began in 1879?

The New Grove Dictionary of Music and Musicians is recognized as a seminal work in the field of Western music reference. First published in 1879, it serves as a comprehensive, multi-volume encyclopedia that covers a vast array of topics related to music history, theory, and practice. The dictionary is renowned for its extensive coverage of composers, styles, genres, and significant works spanning various periods and cultures in Western music. Its authoritative and scholarly approach has made it an essential resource for musicians, researchers, and music educators. The other options listed, while significant in the realm of music literature, do not match the same historical significance or volume count associated with The New Grove Dictionary. The Encyclopedia of Popular Music and The Music Index Online focus on specific genres and resources rather than the comprehensive scope of Western classical music, and The International Index of Music Periodicals serves as a bibliographic resource rather than a detailed reference work.