Floor panels are shown, which are provided with a vertical locking system on short edges including a displaceable tongue that is displaced in one direction into a tongue groove during vertical displacement of two panels. Building panels provided with a locking system for vertical locking of a first and a second building panel by a vertical displacement of the panels relative each other, a displaceable tongue is in a sidewardly open displacement groove provided at an edge of a first panel, said tongue cooperates with a tongue groove provided at an adjacent edge of a second panel for locking the edge and the adjacent edge vertically.