Call numbers are a way of grouping like items together on the shelves.
The Music and Performing Arts Library arranges its scores in two different classification sequences. Those arranged in Library of Congress Classification can be found under letter M, followed by a number (usually four figures, in the case of vocal music); and those arranged according to the Dewey Decimal Classification use just numbers, between 780 and 789 (these still have an M before them and are integrated into the LC). For example:
In Dewey, you will find:
Songs at 784.3, then alphabetically by composer. Books about songs will be found at around the same number in the Dewey sequence.
Opera vocal scores at 782.1, then alphabetically by composer
Oratorio vocal scores at 783.3 (this number also includes some Masses and Requiems)
Anthems and motets are at 783.4, then alphabetically by composer
In Library of Congress Classification, you will find:
Opera full scores at M1500, then alphabetically by composer
Opera vocal scores at M1503, then alphabetically by composer
Masses at M2010/2011 (including Requiems)
Songs at M1613 upwards; song cycles are at M1621.4
Musicals in vocal score are at M1508
Motets at M2020
Cantatas, oratorios etc. at M2023, then alphabetically by composer