This book introduces the process of assigning class numbers by the 23rd edition (2011) of the Dewey Decimal Classification (DDC). The book demonstrates the techniques of number building, involving the use of various auxiliary tables and multiple syntheses. It also discusses the structure of the DDC and the subject analysis of the documents to be classified. Alternative class numbers have also been given whenever official options allow. The graded process of number building mirrors the increasingly faceted nature of the system and its advancing steps towards what S.R. Ranganathan terms as depth classification. To avoid long discussions, the number building has been depicted through equations and facet formulas. The aim is to be amply illustrative and to introduce and teach, step-by-step, all the concepts and methods of the DDC.