Building panels 1, 1′ provided with a mechanical locking system including a tongue 30, at an edge of a first panel 1, cooperating with a tongue groove 20, at an edge of an adjacent second panel 1′, for vertical locking of the building panels. The edge of the first panel is provided with a displacement groove 60, which is downwardly open, and includes an inner wall 61, an outer wall 62, and an upper wall 67. The tongue 30 is formed out of the edge of the first panel. A resilient and displaceable and part 66 of the tongue 30 is displaceable into the displacement groove 60.