A method of describing crashes using both fixed and moving coordinate systems is presented which leads to a theory as to what is the crush zone of a car and, thus, where forward sensors must be placed. Using a moving coordinate system, a car crash performance index is developed which provides a quick method of determining the severity of the car crash pulse and the difficulty fitting an air bag to a particular vehicle. A two-pulse theory is then developed which shows that the vehicle can be divided into the crush zone and the non-crush zone, and the implications of this theory is developed for mounting sensors in each of these locations. The crush zone is that part of the car which crushes during the crash up to the point that sensor triggering is required and the non-crush zone is the remainder of the vehicle. The implications for sensor placement are that non-crush zone sensors must not become part of the crush zone. Crush zone sensors must be in the crush zone. If a car has a soft front structure, a front crush zone sensor is required, and if a car has a rigid structure a compartment sensor is required. Finally, the out-of-position occupant problem is discussed from the point of view of non-crush zone sensor sensitivity. Using the technique of velocity scaling, it is concluded that non-crush sensors should be as sensitive as possible.