on a displaceable frame bearing s while the other end of said arm 10 is also mounted on the attachment arm 11 this construction of a link mechanism 6 7 8 9 10 11 enables the movements and degrees of freedom of the screen and the tabletop to be varied to an extent greater than the actual need by appropriate dimensioning of the units consisting of the manoeuvring device 6 the guide groove 7 the pivotal frame 8 the forwardly located link arm 9 the rearwardly located link arm 10 and the distance between the bearings in the attachment frame 11 with respect to size and placement the guide groove 7 is provided on the fixed stand 5 which is not angled the manoeuvring device 6 and the rearwardly located link arm 10 are fastened in the guide groove 7 the pivotal frame 8 constitutes a fourth link between a rearwardly located link bearing 81 and a forwardly located link bearing 82 this results in a four link mechanism which is adapted for movement by the manoeuvring device 6 while the mechanism as a whole u2014 which is supported by the fourth link u2014 is also pivotal about the frame bearing 8.