U.S Department of Labour
This is a service provided by the U.S Department of Labour that contains the entire SIC code manual. Standard Industrial Classification Codes (SIC Codes) are numerical codes designed by the U.S. Government in order to create uniform descriptions of business establishments.

SIC codes can be used to bring together companies that produce similar products or services. SIC Codes are arranged in a very structured, hierarchical manner :

• The first digit designates a major Economic Division, such as Agriculture or Manufacturing.
• The second digit designates an Economic Major Group, such as Crop Production.
• The third digit designates an Industry Group, such as Grains or Fruits.
  The fourth digit fine tunes the hierarchical structure into a specific Industry.