An air-bag includes a rear panel and a front panel secured to one another to define an inflatable volume. A tether interconnects the panels and has an area of mechanical weakness provided by at least one line of slits or perforations. The tether has a first configuration in which the area of mechanical weakness is substantially intact, and is configured to adopt a second configuration via rupture of the area of mechanical weakness. The first configuration of the tether is effective, during inflation of the air-bag, to limit the maximum depth of the air-bag to a predetermined distance measured between the front and rear panels, and the second configuration is effective to allow an increase in the maximum depth of the air-bag beyond the predetermined distance.