In an impact of sufficient severity, the airbag sensing system detects that the vehicle is in a crash. The sensing system triggers a release of gas from the inflator, which inflates the airbag. The inflator, airbag and related hardware are all part of the airbag modules. Frontal airbag modules are located inside the steering wheel and the instrument panel. For vehicles with seat-mounted side impact airbags, there are also airbag modules in the side of the front seatbacks closest to the door. For vehicles with roof-mounted side impact airbags, there are also airbag modules in the ceiling of the vehicle, near the side window.