Abstract: This paper presents an idiomatic construct for ?-charts which reflects the high-level specification construct of synchronization between activities. This, amongst others, has emerged as a common and useful idea during our use of ?-charts to design and specify commonly-occurring reactive systems. The purpose of this example, apart from any inherent interest in being able to use synchronization in a specification, is to show how the very simple language of ?-charts can used as a basis for a more expressive language built by definitional extension.