Patents provide important technical literature. There are approximately 10 million patents in the U.S. The US Patent and Trademark Office (USPTO) is the official site and answers many questions concerning the patent process. A patent for an invention grants a property right to the inventor, usually for 20 years. The USPTO isn't the best place for patent searching, though, instead search Google Patents or Patent Lens, and the sites listed further below. Both patent applications and granted patents are included.
This image shows the major parts of a patent. Purdue University points to several good tutorials on patent search strategies, including a discussion of patent classification systems. See also this guide's tab on patent classifications.
The Best Patent Web Sites
Cornell Patent Office
More Patent Databases
Patent Searching Prior to 1976
Prior to 1976 patents searching is a bit complicated. While full-text should be available in Google Patents, some has been scanned with OCR - optical character recognition - and there are numerous spelling errors. The print indexes below may be helpful in these cases: