The basic principles of patent searching U.S. patents are issued every Tuesday of the year and have sequential numbers. Currently, there are over 8 million patents, so it is impractical to search in a random manner. With this number of patents, some kind of system must be utilized. In 1830, legislation was passed that required the PTO to keep a record of the patents issued. Prior to this time, patents were issued, but no records were kept of the titles or who received the patents. After 1830, all patents were numbered. At that time, it was realized that a system was needed to separate the patents into categories so that patents belonging to a similar scienti‹c area could be located. A system of classes and subclasses was started and is still in use today.