Every execution (vertical rectangles) has a start and end occurrence. There are three executions -> 3*2 = 6 (de grijze balkjes) Every message (or function call) also has 2 occurrences:  send and receive occurrence. We count 4 calls, each with a send and return -> 4 * 2 = 8 (de message pijlen) Summing up: 6+8 = 14