Currently, the number of available information sources is enormous and still growing. Businesses have databases containing product, client, supplier and payment information. Even at home, people have multiple information sources, e.g. address books on PDAs, mobile phones and desktops. All of these information sources contain information on related items, or
. The user usually only requires the actual information without having to search for it, or be aware that the information is divided over several independent information sources. Integration the information sources would help the user in his or her task, because the information is presented as if it were contained in one information source. As an example, we will use two address books of Mark, who has two address books. One address book is located at home, while the other is located at work. Subsets of these address books are presented in Table 1.