A social group in the widest sense is a number of men and women having in common traits or circumstances which are or which may be, significant for social life. 'Significant for social life' means that they are, or they may become, the bases of common consciousness, of common interests, and of common purposes. This chapter describes the society that distinguished with the three levels of integration: interaction, even if unwilled and unrecognized; mental interaction and willed relationships. The growth of a primary group into a secondary group is usually a matter of great social significance, involving new kinds of relationship and necessitating more organization; failure to observe the transition may create serious difficulties, as is not infrequently seen when staffs of organizations expand rapidly. Many of the marks of social class are passed down as part of familial heritage, and it is therefore true in most societies that social class attaches primarily to families rather than to individuals.