Library classification is a work consists of two steps. Firstly, the "about ness" of the material is ascertained. Next, a call number (essentially a book's address) based on the classification system in use at the particular library will be assigned to the work using the notation of the system. It is important to note that unlike subject heading or thesauri where multiple terms can be assigned to the same work, in library classification systems, each work can only be placed in one class.