In a so called "Conflict of Laws" Private International Law defines the applicable law between States = Which material law shall govern the facts of the matter?
There are different possibilities. Nowadays, there are at least in Europe regulations like Rome I or II which stipulate the same rules for all member states of the European Union. Generally, there are only national regulations to decide on the material law. Often, there are no regulations at all. Then, it is up to the judge to decide on the law to be applied. The following laws could be applied:
- Law of oneself?
- Law of the Court ("Lex fori")?
- Law of the subject matter ("Lex causae")?
- Law of the foreign party?