Commercial vehicle seat with a locking element

The seat has a seat substructure comprising a locking unit (80) for securing unintended rotary motion of a rotary adjustment device for rotating a seat part relative to a body part. The locking unit comprises a translational displaceable mounted locking element (82) i.e. flat slide element, for fixing the rotary adjustment device. The locking element comprises a base body element rotating around a displacement axis (81) where base body height is four times or six times less than base body width. The locking element comprises a groove-spring unit (100).