The role of convention in various definitions of clock synchronization and simultaneity is investigated. We show that two principal methods of synchronization can be considered: system internal and system external synchronization. Synchronization by the Einstein procedure and by slow clock transport turn out to be equivalent if and only if the time dilatation factor is given by the Einstein result (1–v 2)1/2. An ether theory is constructed that maintains absolute simultaneity and is kinematically equivalent to special relativity.