A set of essentially identical panels, such as building panels, provided with a mechanical locking system including a displaceable tongue, which is arranged in a displacement groove with a first opening at a first edge of a first panel. The displaceable tongue is configured to cooperate with a first tongue groove, with a second opening at a second edge of an adjacent second panel, for vertical locking of the first and the second edge. The height of the first opening is greater than a second height of the second opening.