In order to economize on space, in 1972 the Music Division began using the fixed order classification system. In this system, both books and scores are organized by size, then by year, then in order of acquisition. In this system there is no organization by subject.
The initial letter J represents the Music Division. The second letter represents the collection within the division, and the third letter represents the size classification of the item. These are explained in the boxes below.
The three letters that begin a call number are then followed by a two-digit year and an accession number. For example, the call number JMG 89-123 means a volume that is between 30 and 39 centimeters tall which was the 123rd item cataloged in the year 1989.
When the fixed-order system was first put in place, JOB generally referred to items up to 17 centimeters. After 1973 it was realized this was unnecessary, so that all items up to 20 centimeters were classified in the JOC or JPC categories. JOB and JPB came to mean exclusively "laying flat." Since 1973 JOB and JPB have come to represent classification for archival material.
Published materials larger than 49 centimeters are generally assigned Music (Sheet) classmark. See the tab Sheet Music Classifications.