A control device comprises a housing (10) with a least one circularly symmetric control knob (13) recessed into a surface (14) of the housing and mounted for rotation about its axis of symmetry. The knob has a partially exposed circumferential edge (22) at one side of the housing and an external axial projection whose tip (23) has a diameter less than that of the circumferential edge. At least a substantial portion of the external surface (24) of the knob between the tip (23) and the edge (22) slopes outwardly from the tip to the edge, the sloping surface preferably being concave and extending continuously from the tip to the edge. The control device may be a valuator for an interactive graphics terminal, and include a plurality of the said knobs disposed along each of two opposite parallel sides of the housing.