The ten classes and major subdivisions of the Dickinson Classification of Music are:

The scheme is mnemonic.  In class 5, for instance, 52 is the number for string chamber music (5 denoting chamber music, 2 denoting strings); 53 is wind chamber music (5 for chamber music, 3 for winds).

     The average call number consists of five lines.  On the first line, M designates music.  The second line is the classification number; this denotes the original medium of the work.  The third line consists of the number used consistently for the composer, e.g., B39 for Beethoven.  The fourth line describes the piece in hand:  ov for overture, the first letter of a distinctive title, opus or thematic catalogue number.  The fifth line is the first letter of the editor’s or publisher’s name.
Hence:
M                     designates music
52                    string chamber music
B39-4              Beethoven – 4 players
(18/1)               opus 18, number 1
B                      Breitkopf & Härtel edition

     The symbol = is used to show arrangement of a composition.  91=01 indicates a piano-vocal opera score; 85=01 designates a keyboard reduction of the original accompaniment of an oratorio or cantata.          

For many items of music in the catalog, a uniform title is used to bring together all editions, versions, and arrangements of a given composition and all similar collections of the works of one composer.
Titles assigned by the composer or titles of the first edition are considered the standard. For English, French, German, Italian, and Spanish, the original language is used. For some titles in Russian, Czech, Hungarian, etc., the most common English title is used. For languages not using the Latin and Latin-derived alphabets, transliterations from the original alphabets are used.
To locate compositions by medium, “Browse an Alphabetical List” and choose the index “Call Nos. (LC, NLM, Textual).” For example, browsing for M933 and M 933 will retrieve scores of musical shows, while M21 and  M 21 will retrieve scores for compositions for violin and piano.
If a collection of compositions includes all of a composer's works for a given medium, the third line of the call number is "a2" to indicate completeness.

M         Chopin
11        [Piano music]
C45
a2
Sch

If the collection is incomplete, the title so states and the third line of the call number is "a3".

M         Debussy
71        [Vocal music. Selections]
D35
a3
K

For more assistance, please ask a reference librarian.