There are three types of U. S. patents: utility, design, and plant. The utility patent is the most common type of patent as it is the patent which protects unique inventions. Each type of patent has its own numbering scheme.
Utility patents are completely numerical: 5,146,634
Design patents have the letter D before the number: D339,456
Plant patents have the letters PP before the number: PP08,901
Patents are an important and useful form of literature. The most commonly given reason for performing a patent search is a patentability search for "prior art". However, patent literature can be useful for many other reasons, including: