The invention relates to a headbox (1) for a machine for producing a fibrous material web, in particular a paper or cardboard web, comprising a central assembly (2) preferably forming the carrier elements (5, 6) of a turbulence generator (7), said assembly having a table leaf (3) on the bottom side, and further comprising a headbox nozzle (10) adjoining directly downstream in the machine running direction (L) and extending in the machine transverse direction (Q), said nozzle comprising at least one lower and one upper continuous nozzle wall (11, 12) extending over the machine width (B). The headbox (1) according to the invention is characterized in that the lower nozzle wall (11) of the headbox nozzle (10) comprises a lower lip plate (14), in particular a lower lip carrier plate (14.1), which is connected, in particular welded, to the table leaf (3) of the central assembly (2) by means of a plurality of ribs (29) extending in the machine running direction (L) and spaced apart in the machine transverse direction (Q). The ribs (29) are designed in an elastic or pliable manner in the machine transverse direction (Q).