Patent documents are classified worldwide into specific patent classification classes depending upon their technological content. This helps enormously in making the information contained in the millions of patent documents worldwide accessible through searches.

The IPI classifies the inventions in accordance with the International Patent Classification (IPC) schema and also the Cooperative Patent Classification System (CPC):

The International Patent Classification IPC

The International Patent Classification IPC is implemented worldwide by all patent offices. Currently there are around 70,000 different classifications (September 2017). These are regularly updated and refined based on the state of the art. After a classifications revision, patent documents are newly classified according

The official version of the IPC can be accessed via the website of the World Intellectual Property Organization (WIPO) (French or English). The German Patent and Trade Mark Office (DPMA) also provides a useful version available in German (IPC DPMA in German).

International Patent Classification
The IPC is hierarchically structured, as illustrated in the following example:

Pencils with an automatic lead replacement are classified by the IPC as follows:

B43K 21/02:

The search can be refined further with more subgroup with several points.

Cooperative Patent Classification System (CPC)

The Cooperative Patent Classification System is another important classification system.  It was developed as a joint venture between the European Patent Office and the United States Patent and Trademark Office (USPTO), and implemented in 2013. The CPC is based upon the former European classification system ECLA, which is based in turn upon the International Patent Classification IPC. The CPC is more detailed than the IPC which allows for more precise and efficient patent searches. Many patent offices, including the IPI classify patent documents according to the CPC in addition to the IPC.

Other patent classification schemas:

Former US patent classification schema UCLA

Japan Platform for Patent Information (FI und F-Terms)