H.L.A. Hart, answered from the texts and cited to the page.
A legal system is a union of two kinds of social rules — primary rules of obligation, which prescribe or proscribe forms of conduct, and secondary rules about those primary rules.12 There are three kinds of secondary rule. The rule of change identifies the persons and procedures by which new primary rules are introduced and old ones repealed.
The rule of adjudication identifies the persons and procedures by which alleged breaches of primary rules are authoritatively determined. The rule of recognition is the ultimate secondary rule — the social rule, accepted by officials from the internal point of view, that specifies the criteria of legal validity for the system.34 The rule of recognition is not itself a rule that is valid by reference to higher criteria; its existence is a matter of social fact, of what the officials of the system in fact do and how they treat their doing.5
This analysis displaces Austin's command theory of sovereignty, which cannot account for power-conferring rules, cannot account for the persistence of law across changes of sovereign, and cannot account for the legal limits on the highest legislator. It also displaces Kelsen's neo-Kantian Grundnorm, which is presented as a presupposition of legal cognition rather than a social fact about what officials accept.
My debt on this last point is to Hume, not Kant: the normativity of legal rules is to be understood through the attitudes that participants in a legal practice take to their participation, not through a transcendental category.
The union of primary and secondary rules is at the centre of a legal system; but it is not the whole, and as we move away from the centre we shall have to accommodate, in ways indicated in later chapters, elements of a different character.The Concept of Law, p. 100
This requires more detailed attention in the analysis of legal and political concepts than it has usually received. Under the simple regime of primary rules the internal point of view is manifested in its simplest form, in the use of those rules as the basis of criticism, and as the justification of demands for conformity, social pressure, and punishment.The Concept of Law, pp. 98–100
But it is important to see precisely what these presupposed matters are, and not to obscure their character. They consist of two things. First, a person who seriously asserts the validity of some given rule of law, say a particular statute, himself makes use of a rule of recognition which he accepts as appropriate for identifying the law.The Concept of Law, pp. 108–109
He will be of of aware the general likely consequences disobedience: that there are officials who may arrest him and others who will try him and send him to prison for breaking the law. So long as the laws which are valid by the system's tests of validity are obeyed by the bulk of the population this surely is all the evidence we need in order to establish that a given legal system exists.The Concept of Law, pp. 114–115
The assertion that it exists can only be an external statement of fact. For whereas a subordinate rule of a system may be valid and in that sense 'exist' even if it is generally disregarded, the rule of recognition exists only as a complex, but normally concordant, practice of the courts, officials, and private persons in identifying the law by reference to certain criteria. Its existence is a matter of fact.The Concept of Law, pp. 110–111
The union of primary and secondary rules is the central machinery of The Concept of Law and the model of legal positivism in the English-speaking world after 1961. The framework has been extended (MacCormick's institutional theory; Raz's analysis of legal systems and authority; Shapiro's planning theory) and criticised (Dworkin on principles; the post-positivists generally). The standard internal criticism, pressed by MacCormick and Twining, is that the rule-system analysis understates the role of principles, values-as-standards, and analogical reasoning in legal argument — a real point that the framework admits as friendly extension.