  In the analysis of social networks many relatively loose and heuristic
definitions of 'community' abound. In this paper the concept of closely knit
communities is studied as defined by the property that every pair of its
members are neighbors or has at least one common neighbor, where the
neighboring relationship is based on some more or less durable and stable
acquaintance or contact relation. In this paper these are studied in the form
of graphs or networks of diameter two (2-clubs). Their structure can be
characterized by investigating shortest spanning trees and girth leading to a
typology containing just three or, in combination, six types of close
communities.
