Books and periodicals still comprise the main kinds of print materials in most libraries. Bbooks are placed on bookshelves, which are often called stacks, by subject areas. These areas are coded according to a formal classification system. A classification system is a standardized set of rules for labeling and identifying books according to their subjects, authors and other distinguishing information. Each book is given a call number, which is the code that represents: the classification system used; the subject, author, and/or title of the book; and further identifying information. It also serves as the “address” for placing and locating the book on library shelves. For traditional library materials, the selector makes the decision to acquire an item with only limited consultation with other departments following established policies and guidelines.