Abstract: The invention relates to an adapter element for fastening a planar element, in particular an awning, to a vehicle, including an adapter body, which has a first edge region for fastening the adapter element to a vehicle, and a second edge region for fastening a planar element to the adapter element. The first edge region of the adapter body is equipped with a vehicle connection element for releasably fastening the first edge region to a vehicle. The second edge region of the adapter body is equipped at the second edge region with a planar element connection element for releasably fastening the second edge region to an edge region of a planar element. The invention also relates to a modular system having such an adapter element and having a planar element.