Collaboration diagrams are also referred to as communication diagrams. It focuses primarily on the structural aspect of an interaction diagram: how lifelines are connected. A collaboration diagram's syntax is similar to a sequence diagram; the only difference is that the lifeline does not have tails. Each message is numbered to represent the messages transmitted over sequencing. In comparison to the sequence diagram, the collaboration diagram is semantically weak. The object diagram is a subset of a collaboration diagram. Like sequence diagrams, it focuses on the elements rather than the message flow. Because collaboration diagrams are inexpensive, the sequence diagram can be converted directly to a collaboration diagram.