This Organisational system was conceived by Melvil Dewey in 1873 and first published in 1876. It is one of the most widely used in the world.
The DDC as it is shortened to, is used to organise books by subject. Each item is given a shelf mark identifier so it can be located easily. The main subject areas are:
000 Computer science, information, and general work
100 Philosophy and psychology
300 Social sciences
700 Art and recreation
900 History and geography
The first 3 digits are the main subject area. The numbers after the point give a subsection. After the numbers there are 3 letters to represent the author or editor. For example 941.34 DAI This refers to a book on Edinburgh by David Daiches. Books with the same DDC number are shelved first by their number, then in alphabetic order by author or Editor.
If you want a very fun (and most definitely silly) video on how to find a book, this YouTube video is perfect: