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Often a federal department or agency is authorized by Congress to write regulations that flesh out details left unspecified in the U.S. Code. These regulations are also codified, in the Code of Federal Regulations. The numbering is parallel to the U.S. Code numbers.
The Environmental Protection Agency's explanation of the regulatory process, and how you can get involved. Although this is not typically an agency particularly relevant for Social Workers, its explanation of the process is helpful and relevant to the process at other U.S. federal agencies.