A patent family is a set of patents or patent applications in various countries in relation to a single invention, for example when a first application in a country – the priority application – is extended to other countries.
The extended families allow for additional connectors to link other than strictly priority date.
These include: domestic application numbers, countries that have not ratified the Paris Convention, or if the application was filed too late to claim priority.
Another definition, which is broader than the "simple patent family" definition but narrower than the "extended patent family" definition, is to consider that "[a]ll the documents having at least one common priority belong to the same patent family.
"[3] In general, "[p]atent families are [effectively] defined by databases, not by national or international laws, and family members for a particular invention can vary from database to database.