A set of panels including a first panel with a first main plane and a second panel with a second main plane. The first and the second panel are provided with a mechanical locking device for locking a first edge of the first panel to a second edge of the second panel at a junction plane, wherein the first main plane is essentially perpendicular to the second main plane and the junction plane is extending between the first main plane and the second main plane. The first edge includes an edge tongue that extends from the junction plane. The second edge includes an edge groove at the junction plane. The edge tongue is configured to cooperate with the edge groove for locking together the first and the second edges in a first direction which is perpendicular to the first main plane. The edge tongue includes a tongue groove.