Ingeniously applying cutting-edge technology: along the whole profile length, a groove stretches in all four insides of the window frame, into which the necessary fittings can be snapped and screwed together in any place-yet the groove remains invisible! A thin plastic "membrane" keeps it sealed, being penetrated only where the fitting is meant to be during installation. The result: smooth, convenient to clean surfaces all around, no dirt that could collect inside a groove, nothing unsightly perturbing the balanced profile shape.