A window shade for the triangular window of a rear side door of a motor vehicle is provided which includes a guide rail that extends substantially over the entire bottom edge. In the vicinity of the winding shaft, either a pocket is present or the guide rail from there becomes after a short transitional arc a section that runs at a slant. The sliding body of the slide that runs in the guide rail is rotated appropriately to align the connection arm of the slide parallel to the window post.