a door or a window including a stationary primary track a movable secondary track an operating member with one end pivotally fixed in the primary track and an opposite end movable along the secondary track and a guide engageable with a guide surface of the operating member the guide surface is curved along at least a portion of a length thereof such that a tangent to the guide surface at a point where the guide surface crosses a longitudinal center line of the primary track forms an acute angle with the center line the acute angle is substantially constant throughout the range of pivotal movement of the operating member application_number US 70105601 A description in fig1 and 3 of the accompanying drawings there is schematically depicted a mechanism for opening and closing windows the mechanism includes an operating member 1 typically interconnected between a primary slide track 4 and a secondary slide track 6 the primary slide track 4 is intended to be secured to a window jamb whereas the secondary slide track 6 is intended to be affixed or formed integrally with a window frame the near end of the operating member 1 is anchored on the centre line 3 to the primary slide track 4 by a pivot pin 2 the outer end of the operating member 1 is fitted with a pivoted slide 5 which is engaged into the secondary slide track 6 a roller car 9 incorporates guide wheels 14 which keep it constantly aligned with the centre line 3 of the primary slide track 4 a guide roller 7 is affixed to the body of the roller car 9 and is engaged in the slot 8 of the operating member 1 the centre line 10 of the slot 8 in the operating member 1.