Each number stands for a special topic. Every book is given a number and is put on the shelf in number order. Books with the same number are put in alphabetical order by the author’s last name. There are three summaries of the DDC. The first summary contains the ten main classes. The first digit in each three-digit number represents the main class. For example, 500 represents natural sciences and mathematics. The second summary contains the hundred divisions. The second digit in each three-digit number indicates the division. For example, 500 is used for general works on the sciences, 510 for mathematics, 520 for astronomy, 530 for physics. The third summary contains the thousand sections. The third digit in each three-digit number indicates the section. Thus, 530 is used for general works on physics, 531 for classical mechanics, 532 for fluid mechanics, 533 for gas mechanic. A decimal point follows the third digit in a class number, after which division by ten continues to the specific degree of classification needed. A subject may appear in more than one discipline. For example, "clothing" has aspects that fall under several disciplines. The psychological influence of clothing belongs in 155.95 as part of the discipline of psychology; customs associated with clothing belong in 391 as part of the discipline of customs; and clothing in the sense of fashion design belongs in 746.92 as part of the discipline of the arts.