Most materials in the Mudd Library are arranged on the shelf in broad subject areas based on a system called the Library of Congress Classification. These broad areas are indicated by one or two letters at the head of every LC call number. Scores and most sound recordings are arranged under the letter M, books on music under ML and MT. Unlike music, most subject areas do not have mnemonic letters. For example, philosophy books are under B, history under D or E, political science under J. Following the letter or letters beginning the call number is a number indicating a more specific subject area. See LC Classification Guide.
The purpose of the Library of Congress Classification is to help you find what you are looking for in the library. For example, if you are a violinist and would like to browse through the library's collection of string quartets, you could simply look on the shelves under M452, the class for string quartets. Or if you are a pianist and want to find a volume of Mozart piano sonatas, you could go to the M23s and look under M. As you can see, you will find it handy to learn the numbers for your particular performance area. See Introduction to the M Class. See also M class numbers for brass, guitar, organ, percussion, piano, strings, vocal music, and woodwinds, and the very helpful and well-organized Subject Guide to Library of Congress Music Classification Numbers from the University of Tennessee music library, which includes ML and MT as well as M class numbers.