Folding-arm bearing for an oscillating-swinging leaf of doors, windows or the like, comprises a support inserted on the sash-frame and a square-shaped blade having a wing onto which is pivotally mounted a plate bearing a folding-arm leg by a connector. In order to facilitate the reversibility of this folding-arm bearing without disassembling of components, the wing of the blade is provided, on its upper and lower edges, with a pin engaging into an opening made in the connector, and the plate is mounted onto this wing by means of an eccentric having a rotation which generates a vertical movement of the plate causing the disengagement of a pin from the opening present in the connector.