The ordering rules are: 1.  A message end must occur after the message start. This rule is basic CAUSALITY. 2.  Occurrences on a single lifeline are time-ordered from top-to-bottom. This rule is the definition of the lifeline. This rule enforces a strict ordering of occurrences on the lifeline. 3. The return, if any, from a message, can only occur after the original message is received. This rule is a consequence of rule 1 and rule 2