Abstract: A device for sealing a gap between a door and a floor or between a window and a window frame comprises a deformable sealing element that seals the gap when the door or window is closed, and an L-shaped activation rail attached to the sealing element and having a vertical shank and a horizontal shank. The sealing element is attached at a free end of the horizontal shank. The activation rail engages with a door frame, or window frame. The activation rail is pivoted when the door or window is being closed and causes the sealing element to engage the floor or window frame. There is an attachment rail for affixing the sealing element to a door or window and arranged on a first side wall of the attachment rail. The attachment rail has an L-shaped cross section, and the sealing element is arranged between the activation rail and a second side wall of the attachment rail. The activation rail is joined to an upper end of an upright-standing shank of the attachment rail.