The United States Patent and Trademark Office (USPTO) was established under Article 1, Clause 8 of the United States Constitution:
"The Congress shall have Power To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries."
It is an agency under the U.S. Department of Commerce.
Manual of Patent Classification(http://www.uspto.gov/web/patents/classification/) -- Listing and description of the system of U.S. classification codes used for searching patents by topic. This includes both the old U.S. patent classifications codes (USPC) and the new Cooperative Patent Classification codes (CPC).