Hello, While using the Boost.StateChart library, I noticed strange behaviour when orthogonal states are used. Supposed I have a state machine with state A, which contains a state B, and state B has two orthogonal states C and D. Suppose A handles some event E, and D is able to handle the same event E. Now if the statemachine is in state C and D and I send event E, this can happen: E is sent to C, which is unable to handle E. E gets forwarded to B, and then to A. There, it is handled by A. I would expect this behaviour: E is sent to C, it is unable to handle it, and then the event E is forwarded to the next orthogonal state in B, which is D. Then, D is able to handle it. This way, event E ends up handled by D instead of by A. The statechart reference states the current behaviour as the correct behaviour: Class template state_machine, void process_event specifies in point 3: "Select an arbitrary but in this reaction search not yet visited state from all the currently active innermost states." and in 4a: "Searches a reaction suitable for currentEvent, starting with the current innermost state and moving outward until a state defining a reaction for the event is found." So, the implementation matches the specification. But what is the reason that it is not specified that first all orthogonal states are tried before moving outward in search for a reaction? Is this the way the UML standard specifies it? I tried to read the UML standard, but I can't find something relevant for propagating events. best regards, Richard Peters This message and attachment(s) are intended solely for use by the addressee and may contain information that is privileged, confidential or otherwise exempt from disclosure under applicable law. If you are not the intended recipient or agent thereof responsible for delivering this message to the intended recipient, you are hereby notified that any dissemination, distribution or copying of this communication is strictly prohibited. If you have received this communication in error, please notify the sender immediately by telephone and with a 'reply' message. Thank you for your co-operation.