A foldable device has a hinge to allow it to be opened or closed. The hinge has two shafts separately secured to two panels of the foldable device. Each shaft has a coupling section with a recess on the shaft surface. A substantially planar spring having two curved sections is used to join the shafts through the corresponding coupling sections. Each curved section has a protruding portion on the inner diameter of the curved section. When the foldable device is in the closed position, the protruding portion in the second curved section is engaged in the recess of the second shaft, while the first curved section is disengaged from the recess of the first shaft. The foldable device can be opened toward a locking position where both protruding portions are engaged in the respective recesses. At any position, the hinge is rotated around only one of the shafts.