If the message line is slanted, then the primed-name indicates the time at which the message is received. Constraints may be specified by placing Boolean expressions in braces on the sequence diagram. In case of a message from an object to itself, the arrow may start and finish on the same object symbol. The arrow may also be labeled with a sequence number to show the sequence of the message in the overall interaction. Sequence numbers are often omitted in sequence diagrams, in which the physical location of the arrow shows the relative sequences, but they are necessary in collaboration diagrams.
Sequence numbers are useful on both kinds of diagrams for identifying concurrent threads of control.
A message may also be labeled with a guard condition. A procedure call is drawn as a full arrowhead. Normally message arrows are drawn horizontally.