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 at a first edge of a first panel. A second panel is provided with a tongue groove at a second edge. The displaceable tongue is configured to cooperate with the tongue groove for locking together the first and the second edge. The displaceable tongue has a spring constant that varies along the length of the tongue.