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A mechanism for automatically blocking the gap between the bottom edge of a door leaf, hung to open and close within a surrounding frame, and the adjacent threshold, when the door is closed; and automatically unblocking that gap when the door starts to open again, comprises an elongate bar housed above a strip of material in the bottom edge region of the door leaf. The bar and strip of material each are coupled to respective attraction/repulsion elements. One element is a magnet and the other is a piece of material having a positive reaction to a magnet, typically a further magnet. One end of the bar or an auxiliary member coupled to the bar protrudes from an edge of the door adjacent the door frame. The arrangement is such that, with the door open, the strip of material is raised clear of the adjacent threshold. As the door closes, the protruding end of the bar or auxiliary member contacts the upright of the door frame and is moved longitudinally to take the two sets of magnets out of line with one another and to allow - or to positively repulse - the strip of material to fall and block the gap between the bottom edge of the door leaf and the adjacent threshold.