A zip slider has: a slider body having upper and lower elements providing two entry channels through which zip teeth pass and slider un/mating; a pull tab having a cam; a biasing spring having a locking prong and a follower surface bearing against the cam. The cam on the follower surface biases the tab into a closed position in which it lies against the upper element. When the tab is in a lifted position, the cam action on the follower surface raises the prong above the mated teeth, enabling relative motion of the teeth and body; when the tab is in the closed position, the prong bears against the teeth to prevent unwanted relative slider and teeth motion. The slider has a counter bias, acting between the upper element and the tab to apply a biasing force on the tab acting against the action of the cam and follower surface.