Abstract: In a tiltable or pivotal window in which the frame and sash side members that are perpendicular to the axis of rotation have oppositely facing abutment surface sections on either side of the axis area, said abutment surface sections merge smoothly into each other through an abutment surface section formed as a helical surface of about 180.degree.. A continuous weather strip may be provided on one or both of the abutment surfaces. If such strips are provided on both surfaces, they may be magnetic with opposite transverse polarization to improve tightness and minimize sliding contact between the strips in the helical surface area.