Card catalogues have been replaced by automated catalogues in the library world. The data that earlier figured on card catalogue records, including UDC numbers, together form the database records. The use of UDC can be difficult because in order to determine the meaning of a number, we have to look up its natural language definition in the tables. Synonyms - terms having identical meanings but different forms - need to be related. Homonyms - terms having different meanings but identical form - need to be separated. With the creation of cross references it would be possible to avoid ambiguity in information retrieval languages as well as to keep track of the changes made in UDC numbers.