Norms are the informal rules that groups adopt to regulate members' behaviour. Norms refer to what should be done and represent value judgments about appropriate behaviour in social situations. Although they are infrequently written down or even discussed, norms have powerful influence on group behaviour. They are a fundamental aspect of group structure as they provide direction and motivation, and organize the social interactions of members. Norms are said to be emergent, as they develop gradually throughout interactions between group members. While many norms are widespread throughout society, groups may develop their own norms that members must learn when they join the group. There are various types of norms, including: prescriptive, proscriptive, descriptive, and injunctive.