Transitions provide the links between activities. They are the primary flow control construct and dictate the order in which activities will be executed. Transitions are unidirectional and an activity can have multiple outgoing and incoming transitions forming branch and synchronization points respectively. Since every process definition must have one start and one end activity (see Base Activity), a process definition can be thought of informally as a directed graph in which activities are the vertices, transitions are the arcs and every path from the start activity eventually leads to the end activity.