The call numbers' for a document consists of three elements, viz.,class number, book number and collection number. The following example will show the presence of these three elements in a call number. Jawaharlal Nehru: An Autobiography with Musings on Recent Events in India, Bombay, Allied. 1962. The call number for this book by Dewey Decimal classification is:
Thus, with the help of book number and collection number a document is fully individualised. It means that the call number of a document is unique. Different methods are in use to devise book number and collection number. It is left to individual 'libraries to follow one of these methods or, if possible, devise their own method or practice.