A headbox contains a nozzle channel which, in the direction of flow of the stock suspension or pulp. converges towards an outlet gap or slice. The nozzle channel contains two lips. At least one of these lips has a wall portion or section which is concave with respect to the stock suspension. This wall section is defined by imaginary curved reference lines. This wall section or portion is formed by steps or step portions extending essentially parallel to the outlet gap. Each of the step portions comprises a substantially planar partial surface extending comparable to the direction of flow of the stock suspension and an end surface disposed transversely with respect to its related planar or flat partial surface. At the region of this particularly simple to fabricate wall section there is thus produced microturbulence directly prior to the sheet formation. Such microturbulence ensures for a uniform formation of the stock fibers which are suspended at the region of the marginal flow or boundary region of the stock suspension.