International Patent Classification (IPC)
Most patent offices use this system as the primary or secondary classification. Consists of over 70,000 classifications.
U.S. Patent Classification (USPC)
System used by the USPTO to classify patent documents. Contains 450 classes and 150,000 subclasses.
European Classification (ECLA)
Based on the IPC system and used by the EPO to classify patent documents. Contains about 140,000 subclasses.