First, most legal institutions exist by virtue of legislative rules. Secondly, legislation can only take place within the framework of a legal institution. To clarify what legal institutions are, attention is paid to MacCormick's distinction between abstract institutional legal concepts on the one hand and particular legal institutions exemplifying such concepts on the other. MacCormick's classification, consisting of institutive rules, consequential rules, and terminative rules of legal institutions, is extended with three additional classes, namely, constitutive rules, content rules and invalidating rules. A test shows that complete legislation constituting a certain class of legal institution contains rules of all distinguished classes.
Legislation takes place by appeal to power-conferring rules that form part of a legal institution.
Modern legal systems are characterized as nested sets of legal institutions that are connected by the exercise of power-conferring rules. The component parts of legal institutions are classified. Attention is paid to the manner in which legal institutions are socially accepted. Lagerspetz's conventionalist method of grounding the social existence of legal institutions on shared beliefs is critically examined and rejected. It is concluded that the classical idea that legislation would mainly consist of general rules of conduct is far too simplistic.