Two or more primitive changes are said to be concerted (or to constitute a concerted process) if they occur within the same elementary reaction. Such changes will normally (though perhaps not inevitably) be 'energetically coupled'. (In the present context the term 'energetically coupled' means that the simultaneous progress of the primitive changes involves a transition state of lower energy than that for their successive occurrence.) In a concerted process the primitive changes may be synchronous or asynchronous.