In libraries, the card catalog is the index. The three indices are used in the library. The Author, Subject and title are the 3 indices. Each index is relative. The index permits the user to find the book according to the feature of the book. Separately from the feature the indices also give the number or the position of the book.
To search for a book, we seem into any of the indices and find the number. With this number, we can locate the shelves containing books in the range of the number we are searching for. Subsequent to the particular shelf is located, following is the lexical order of the number, we locate the book.
Actually, the book is retrieved from the system of the storage. Theoretically, the book is found logically from the system of retrieval. The numbers and shelves are the storage system. The card indices are the recovery system. The books are acknowledged with one system and retrieved with the help of the other system. The customer specifies the book by using the retrieval system. The librarian recovers the book by using the storage system. The retrieval systems are also based on relative search. If the relative retrieval system remembers facts like