An adjustment system for a seat has a first bearing shell for receiving a seat surface and/or a seat backrest, a second bearing shell for connecting to a substructure, a first wedge-shaped adjustment ring, which is rotatably mounted on the first bearing shell, a second wedge-shaped adjustment ring, which is rotatably mounted on the second bearing shell and rotatably mounted on the first adjustment ring, a first drive for rotating the first adjustment ring relative to the second adjustment ring, a second drive for rotating the second adjustment ring relative to the second bearing shell, and a blockade mechanism for electively connecting the first bearing shell and the second bearing shell for conjoint rotation.