If A and B are separate sets (disjuncts), and I is a double (binary) connection connecting A and B, then S = (A, B, I) is the
structure of incidence. This relation is also known as the incidence relation. The two components of a A are referred to as community dots and are denoted by lowercase letters of the alphabet, whereas the community blocks (or straight lines) of a X are denoted by capital letters of the alphabet.