The fitting part comprises a mounting or insertion aid for fitting or attaching a window sash at a frame (17). A swiveling part is connected to a frame on which a window sash is fitted. The mounting or insertion aid comprises a slanting lateral element (20) that is provided with a sash retainer (21). The window sash is guided on the sash retainer in an inclined position along its sash part (3) or a lower horizontal sash profile.
