In a multi-ply headbox, the nozzle chamber is subdivided by rigid partition walls into a number of nozzle chambers, and the slice openings or outlet slices are adjustable independently of one another. This is accomplished by providing a pivotable mounting of the partition wall or walls in a guide block, for instance, such pivotable mounting being accomplished by a substantially cylindrical shell or cup bearing structure. The desired pivotable movement or adjustment of the rigid partition wall can be undertaken by an inclined groove-cam guide arrangement effective at a projection of the partition wall or by pressure cushions, such as pressure hoses effective at both sides of the projection.