A headbox for a papermaking machine has an outlet channel having the width of the machine which is limited by a rigid channel wall and by a movable channel wall. The movable channel wall is part of a channel-wall support. Between the latter and a supporting member, there is a pressure cushion which counteracts the pressure prevailing in the outlet channel. A lift device is swingably mounted, at one end, on the rigid headbox housing and, on the other end, on the channel-wall support. In each of the two end walls of the channel-wall support, there is provided a spherical joint, the center point of which lies in a plane extending through the end point of the channel width. The lift device is coupled to the spherical joint by means of a C-shaped spindle head.