Abstract: A three-layer headbox has two rigid separator vanes (11; 12) mounted in the headbox slice chamber (10) to form two outer stock flow channels (39; 41) and an intermediary one (40). The upstream end of each vane (11; 12) is securely fixed in cantilever fashion and its downstream end (15; 16) is unattached and free and provided with a vane extension (17; 18). Also the downstream end (20; 22) of the extension (17; 18) is unattached and free and is located just downstream of the slice opening (14). The vane extension (17; 18) is thinner than the vane (11; 12), so that a step (23, 24) is formed on each side of the vane (11; 12) and extension (17; 18) assembly. To improve the layer formation, each vane (11; 12) and each vane extension (17; 18) has a portion located in a converging downstream portion (13) of the slice chamber (10), and the vane portions and the extension portions are of substantially equal length.