Abstract: A lateral door opening of a motor vehicle is delimited at the bottom by a side sill, which exhibits a supporting structure and an outer side panel. The outer side panel is hinged to the supporting structure by at least one quadruple joint so that the side panel can move from an effective position, in which it improves the aerodynamic properties of the motor vehicle, into a boarding position, in which the side sill is narrower than in the effective position from the perspective of the transverse direction of the vehicle, so that it is easier for a passenger to get in and out through the door opening as the passenger's legs have to be raised over the side sill when getting in and out. The narrower the side sill, the easier it is to get in and out. The functions of the supporting structure of the side sill with respect to rigidity and crash worthiness are not affected.