This paper shows how the communication-diagram mechanism can be supplied with concepts, so that each collaboration can be specified. It enriches sequence expressions of messages with sub-terms that denote: Types of control structures choice, concurrence, and repetition of messages; and Complete information about their guarded branches, parallel threads, and iterative messages. Furthermore, each concatenation of these sub-terms maps onto an interlacement of message structures in an abstraction level or within levels of abstraction.