Transients in combinational circuits are approached through an analysis of the transition process in a pure delay circuit model that completely separates delay from structure and logic. During any transition, only monotonically increasing sequences of transient input vertices can occur because no transient input variable can change more than once. This restriction to the shortest trajectories through the transition subcube yields a common criterion for all types of hazards in feedback-free combinational circuits: a transition is hazard-free iff the transient switching function is unate in the transition subcube with respect to the initial and final vertices.