A set of essentially identical panels (1, 1′), such as building panels, provided with a mechanical locking system including a displaceable tongue (30), which is arranged in a displacement groove with a first opening at a first edge of a first panel (1). The displaceable tongue is configured to cooperate with a first tongue groove (20), with a second opening at a second edge of an adjacent second panel (1′), 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.