Colleges and large public libraries in the United States use the LC Classification System combining letters and numbers. This allows for more combinations with which to identify and organization information by subject. Call Numbers are used to keep materials on the same subject together on the shelves. In reading the LC Call Number, the book is found first alphabetically in the F section on the book shelves, then the number 184 will be shelved between F183 and F185. The second row is again read alphabetically looking for the C, but the number 28 is read as the digits 2 and 8.
Information professionals assign only one subject number to each book, periodical or AV material.
This means that while other subject content may be included in the book, only one of the subject areas will be represented by the call number.