Dewey decimal book classification was first published in 1876. After 22 major revisions, the content has been revised and expanded to a considerable extent. The latest edition is the 2004 edition. This classification uses three digits to indicate the classification code, and * * * can be divided into 10, 100 and 1000. In addition to the three-digit classification, there are generally two additional codes to indicate the discussion of different regions, time, materials or other characteristics. The classification code and the additional codes are separated by a decimal point ".". For example, 330 table economics +.9 table difference discussion +.04 table Europe = 330.94 table European economics.