H.L.A. Hart famously suggested to understand legal systems as consisting not only of primary rules – rules that require or forbid certain conduct – but also of secondary rules – rules that determine how rules are created, changed, and abolished. This is enough for law within one system, but it cannot account for relations between different legal systems. For this purpose, the chapter introduces the concept of tertiary rules that determine a legal system’s relation with another legal system. The most fundamental such tertiary rule is the rule of external recognition – the rule with which a legal system recognizes a foreign law as law. But this is not the only tertiary rule – choice-of-law rules and certain other rules also count as tertiary rules. The chapter demonstrates the empirical and theoretical importance of the concept of tertiary rules and discusses other examples.