The invention relates to method of manufacturing a recess wall lined with bristles for a mouthpiece for simultaneously brushing at a plurality of dental positions, and to a recess wall and mouthpiece manufactured according to the method. In a providing step, an elongate, continuous, bristled sheet part having a first side lined with a plurality of bristles is provided. The elongate sheet part defines a length axis extending parallel to the sheet part and, viewed transverse to the length axis, a cross-sectional shape. The sheet part obtained in the providing step has an initial condition in which the cross-sectional shape has an initial shape, and the length axis is arch-shaped. In a transforming step, the sheet part of the providing step is transformed from the initial condition to a final condition to provide the recess wall. In the final condition the cross-sectional shape has a final U-shape, the length axis is arch shaped, and the first side is a concave inner side of the final U-shape. The first side is in the final condition more concave than in the initial condition.