What are they?
If there are several applications or publications for an individual invention (in other countries), claiming the same priority or priorities, we talk about a "patent family". All of these "family members" are related to one another by common priority numbers with associated priority dates.
The concept of the patent family first emerged through the Paris Convention on the Protection of Industrial Property in 1883, while automated systems enabling patent family searching became available through the establishment of the IIB in The Hague in 1947 and INPADOC in Vienna in 1972. Since then, patent searching has evolved due to exponential improvements in computing and communications technology.
The term patent family can be defined in a number of ways, depending on the relationship between a patent document and its priority or priorities within the meaning of the Paris Convention. The differences only become obvious when the structure of a patent application is complex, i.e. when applications are filed in several countries. Such applications may cite various earlier applications as priorities, or the diverse patent offices involved in the grant process may accept or refuse different patent claims. This results in patents which have different scopes of protection.
An important point when using any database to retrieve information on patent families is that there is never any guarantee that you will find all the corresponding patent documents that exist. Database producers do what they can to ensure completeness, but they can never guarantee it.